Book before Jeremiah (6)
I believe the answer is:
'book before jeremiah' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for isaiah that I've seen before include "Biblical Book of Prophecies" , "Biblical prophet (and his book)" , "After Psalms, the longest book in the Old Testament" , "Book of prophecies in the Bible" , "Major Hebrew prophet and O.T. Book" .)