In Latin, a humorous book (5)
I believe the answer is:
nahum
'book' is the definition.
(I know that Nahum is a book)
'in latin a humorous' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NAHUM' is hidden within 'latin a humorous'.
(Other definitions for nahum that I've seen before include "Destruction-of-Nineveh prophet" , "Hebrew prophet - Biblical book" , "Minor prophet" , "Prophet's book" , "Old Hebrew prophet" .)