Adult writing about love in book (4)
I believe the answer is:
amos
'book' is the definition.
(Old Testament book)
'adult writing about love' is the wordplay.
'adult' becomes 'a' (abbreviation).
'writing' becomes 'ms' (abbreviation for manuscript).
'about' is an insertion indicator.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'ms' enclosing 'o' is 'mos'.
'a'+'mos'='AMOS'
'in' acts as a link.
(Other definitions for amos that I've seen before include "Old Testament book" , "30th OT book" , "Biblical prophet and OT book" , "American wild dog" , "Hebrew prophet" .)