One’s very large book of prophecies (4)
I believe the answer is:
amos
'book of prophecies' is the definition.
(I've seen this in another clue)
'one's very large' is the wordplay.
'one's' becomes 'am' ('one is' can humorously mean 'I am').
'very large' becomes 'os' (abbreviation for oversize).
'am'+'os'='AMOS'
(Other definitions for amos that I've seen before include "Prophet in the Bible" , "man" , "Biblical prophet and OT book" , "Old Testament prophet" , "30th OT book" .)