Book revealing bitterness about a bighead (5)
I believe the answer is:
bible
'book' is the definition.
(I know that book can be written as Bible)
'bitterness about a bighead' is the wordplay.
'bitterness' becomes 'bile' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'a bighead' becomes 'b' (the head letter of 'big').
'bile' going around 'b' is 'BIBLE'.
'revealing' acts as a link.
(Other definitions for bible that I've seen before include "Book of the Scriptures" , "Book regarded as the ultimate authority on its subject" , "Holy books" , "An authoritative text" , "work that's accepted" .)