In what sense is the bible is "clear"/perspicuous?

Sometimes it helps to understand the words one uses. Like cult (religious veneration of someone or to something) or perspecuity.

I won't delve into the word cult or the problems of using Traditions that so often conflicts with other Traditions, but instead will focus on the OP's conundrum of the word perspecuity. It is not saying all things Biblical are understandable, but just those things necessary for salvation.

7. All things in Scripture are not alike plain in themselves, nor alike clear unto all; yet those things which are necessary to be known, believed, and observed, for salvation, are so clearly propounded and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them. -source-

Confusion as to what the word means probably arises from those whose Traditions are not clearly found in Scripture. But again I digress; just hope to point out something. So, let's see if there is any confusion to salvation.

But we [Jewish] believe that through the grace of the Lord Jesus Christ we shall be saved, even as they [Gentile believers]. Acts 15:11

That was Peter speaking. Here is Paul

For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Eph 2:8

Now it would be probably someone's Tradition brought to the table of salvation that muddies the clarity of Scripture that says we are saved by grace through faith in Christ Jesus, but for us who trust God's witnesses, Scripture, as records the two apostles Peter and Paul, is clear enough.

II Peter 3:14-16 (RSV)

14 Therefore, beloved, since you wait for these, be zealous to be found by him without spot or blemish, and at peace. 15 And count the forbearance of our Lord as salvation. So also our beloved brother Paul wrote to you according to the wisdom given him, 16 speaking of this as he does in all his letters. There are some things in them hard to understand, which the ignorant and unstable twist to their own destruction, as they do the other scriptures. (Bold is mine)

The Bible's central message (the gospel, the covenants, GOD's love, Jesus' purpose) is plain to those who seek that message. However, Peter here tells that the Bible and its central message is not immune to twisted interpretation. This is demonstrated in Jesus' dealings with the religious leaders of His day and in Paul's references to man's refusal to acknowledge the truth of GOD's sovereignty over creation. The message is clear, but can be twisted.

If I understand correctly, the implication of the clarity of scripture is that the Bible is the what the Church uses to teach, reprove, correct, and train itself. The Church isn't just naturally always correct and won't just naturally stay correct without that.

Theopedia https://www.theopedia.com/clarity-of-scripture

"the meanings of the text can be clear to the ordinary reader, that God uses the text of the Bible to communicate His person and will."

..."The witness of the Church throughout the ages is that ordinary people, who approach it in faith and humility, will be able to understand what the Bible is getting at, even if they meet with particular points of difficulty here and there."

Westminster Confession of Faith http://www.ccel.org/ccel/anonymous/westminster3.i.i.html

[6.006] 6. The whole counsel of God, concerning all things necessary for his own glory, man's salvation, faith, and life, is either expressly set down in Scripture, or by good and necessary consequence may be deduced from Scripture: unto which nothing at any time is to be added, whether by new revelations of the Spirit, or traditions of men. Nevertheless we acknowledge the inward illumination of the Spirit of God to be necessary for the saving understanding of such things as are revealed in the Word; and [PCUS that] there are some circumstances concerning the worship of God, and the government of the Church, common to human actions and societies, which are to be ordered by the light of nature and Christian prudence, according to the general rules of the Word, which are always to be observed.

[6.007] 7. All things in Scripture are not alike plain in themselves, nor alike clear unto all; yet those things which are necessary to be known, believed, and observed, for salvation, are so clearly propounded and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them.

Wikipedia

According to Wikipedia, it's the position that through reading the Bible, even the unlearned can become justified with God.

Lutheran

Wikipedia then goes on to give Lutheranism as an example. Even though I'm not Lutheran, I find the statement to be quite the way I think of it:

Lutherans hold that the Bible presents all doctrines and commands of the Christian faith clearly. God's Word is freely accessible to every reader or hearer of ordinary intelligence, without requiring any special education. Of course, one must understand the language God's Word is presented in, and not be so preoccupied by contrary thoughts so as to prevent understanding. As a result of this, no one needs to wait for any clergy, and pope, scholar, or ecumenical council to explain the real meaning of any part of the Bible.

Note that the Lutheran statement makes the points that "all doctrines and commands" are presented clearly. That stands in clear opposition to the Roman Catholic and Orthodox traditions. It is also quite provocative; after all, even protestants have traditions that may not be directly implied by scripture.

Excerpts from https://www.tms.edu/m/tmsj15i.pdf (THE PERSPICUITY OF SCRIPTURE by Larry D. Pettegrew, Prof. of Theology, The Master's College)

“Protestants hold that the Bible, being addressed to the people, is sufficiently perspicuous to be understood by them, under the guidance of the Holy Spirit; and that they are entitled and bound to search the Scripture, and to judge for themselves what is its true meaning.” - Charles Hodge

“The Scriptures are in such a sense perspicuous that all that is necessary for man to know, in order to his salvation or for his practical guidance in duty, may be learned therefrom, and that they are designed for the personal use and are adapted to the instruction of the unlearned as well as the learned.” - A A Hodge

Scripture can be and is read with profit, with appreciation and with transformative results. It is open and transparent to earnest readers; it is intelligible and comprehensible to attentive readers. Scripture itself is coherent and obvious. It is direct and unambiguous as written; what is written is sufficient. Scripture’s concern or focal point is readily presented as the redemptive story of God. It displays a progressively more specific identification of that story, culminating in the gospel of Jesus Christ. All this is to say: Scripture is clear about what it is about. - James Patrick Callahan

The doctrine of Perspicuity is offered to counter the view of the Catholic church that special teachers and clergy are required for the average Christian to usefully interpret scripture. It is also offered to counter post-modern hermeneutical trends that import complex and obscure ideas into the text.

A scripture verse that most clearly defines it for me is this:

“This is the covenant I will make with the people of Israel
    after that time,” declares the Lord.
“I will put my law in their minds
    and write it on their hearts.
I will be their God,
    and they will be my people.
No longer will they teach their neighbor,
    or say to one another, ‘Know the Lord,’
because they will all know me,
    from the least of them to the greatest,”
declares the Lord.
“For I will forgive their wickedness
    and will remember their sins no more.” - Jeremiah 31:33-34

The doctrine does not deny that hard hearts, sinfulness, confusing foreign ideas or inattentiveness can prevent a person from gaining clear understanding, or that the ministry of the Holy Spirit is not required, or that clergy, professors and the like are unprofitable if consulted. However, what it does say is that diligent study, by a person of ordinary intelligence, will in time result in a clear understanding of the meaning of the Bible, and that when that meaning is finally understood, it will be plainly visible in the text, not some esoteric or exotic meaning found in equidistant letter codes or numerological nonsense or extreme use of typology and allegory that so distorts the meaning as to be an entirely other text stamped upon the words that are really there. The experience is one of seeing what was there all along and knowing it was there, even if the meaning is surprising.

This is probably a "no win" situation attempting to answer such a question and this will, in no way, be considered a comprehensive answer for such a simple question that has been so complicated by so many people with so many varying agendas. Many years ago, one of my Bible professors stated that the best commentary on the Bible is the Bible. In the nearly 40 years past since then, I have found this to be, "a faithful saying". That being said, I would like to address this question initially with what God has to say about it:

"The wrath of God is being revealed from heaven against all the godlessness and wickedness of people, who suppress the truth by their wickedness, 19 since what may be known about God is plain to them, because God has made it plain to them. 20 For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that people are without excuse." (Rom. 1:18-20)

Now this passage is referring to 'natural revelation' and not particularly to Scripture but can certainly be construed as establishing a pattern. If His eternal power and divine nature which can be "clearly seen" through His creation, is it such a leap to consider that it is equally clear through His word, His divine revelation? The questioner is questioning the how "clear" the Bible actually is. Peter asserts, " We also have the prophetic message as something completely reliable, and you will do well to pay attention to it..." (2 Peter 1:19). He also asserts, " Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation of things. 21 For prophecy never had its origin in the human will, but prophets, though human, spoke from God as they were carried along by the Holy Spirit." (2 Peter 1:20).

Let's take a moment to consider the "Romans Road to Salvation" (the first part of which has already been noted), as an example, to gauge a level of clarity:

"For all have sinned, and fall short of the glory of God." (Rom. 3:23), "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord." (Rom. 6:23), "But God demonstrates His love toward us, in that, while we were still sinners, Christ died for us." (Rom. 5:8) and "That if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation. (Rom. 10:9,10).

Now one may dispute this, but these verses are pretty clear in their message. There are certainly passages which are somewhat more ambiguous. Here is one that has puzzled me for many years, "There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown." (Gen. 6:4). In this case the ambiguity is not so much in the text but in the questions it raises.

Now, in light of my initial statement, consider this: " For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened. 22 Although they claimed to be wise, they became fools 23 and exchanged the glory of the immortal God for images made to look like a mortal human being and birds and animals and reptiles.

24 Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. 25 They exchanged the truth about God for a lie, and worshiped and served created things rather than the Creator—who is forever praised. Amen."

Based on this information and years of experience, I feel comfortable making the following statements as to the clarity of the Bible.

1. The message of the Bible is clear.
2. Many verses in the Bible are not so clear.
3. God Himself has stated that there are those who purposely exchange "the truth about God for a lie...)
4. When someone says, "The Bible is clear on this topic.", the legitimacy of that statement would depend greatly on the topic.
5. Though the Bible is clear in its message, the continual efforts of those who would 'put a lie in its place' or interpret it to their own benefit have confused many.
6. I would not want to be one of the culprits referred to in verse 5.