French composer in row with philanderer (9)
I believe the answer is:
libertine
'philanderer' is the definition.
(thesaurus)
'french composer in row' is the wordplay.
'french composer' becomes 'ibert' (I've seen this before).
'in' is an insertion indicator.
'row' becomes 'line' (I've seen this before).
'ibert' put inside 'line' is 'LIBERTINE'.
'with' acts as a link.
(Other definitions for libertine that I've seen before include "Roue" , "Sensually indulgent person" , "Say, Don Giovanni" , "reprobate" , "Morally dissolute" .)