I've been reading Turretin on perspicuity of Scripture, and found it interesting the extent to which he explicitly qualifies the question. That there were such qualifications I had already known (and have argued here once or twice), but Turretin lays it out very nicely. He identifies the following as what is not meant by 'perspicuity of Scripture':
(1) It does not mean that Scripture is clear and easy to interpret for everyone; interpretation of Scripture requires illumination of the Spirit. So it's not perspicuity on the part of the subject (the reader) but on the part of the object (Scripture).
(2) It does not mean that there are no mysteries in Scripture, i.e., content so sublime that human understanding cannot exhaust it, and could not be known by us at all without divine condescension. So it's not perspicuity on the part of the content but on the part of the way it is presented.
(3) It does not mean that no interpretation or exposition is ever required; not everything in Scripture is equally clear, and some things require profound study. So it's not universal perspicuity, but perspicuity in things necessary to salvation.
(4) It does not mean that matters necessary to salvation are clearly presented wherever they are found.
(5) It does not mean that interpreting Scripture involves no necessary means for interpretation (and among these Turretin includes, at least for all cases not involving special divine intervention, "the voice and ministry of the church, lectures and commentaries, prayers and vigils").
So that's what it is not. What, then, is it? Turretin puts it this way: Scripture is understandable
(1) in matters necessary for salvation
(2) not with regard to the reader but with regard to Scripture itself
(3) in such a way that it can be read and understood salutarily without appeal to external traditions.
And he argues that this is true on the basis of
(1) Scriptural statements of lucidity;
(2) God's being the Father of lights;
(3) Scripture's purpose being to serve as canon and rule of faith and morals;
(4) The actual ease of understanding the basic message;
(5) Its function as a covenant or treaty between God and ourselves.
A very interesting discussion, and recommended for those who occasionally find themselves, willy-nilly, in sola-scriptura wars.