Convenience, joining fellow libertine (6,3)
I believe the answer is:
ladies man
'libertine' is the definition.
(I've seen this before)
'convenience joining fellow' is the wordplay.
'convenience' becomes 'ladies' (I've seen this before).
'joining' is a charade indicator (letters next to each other).
'fellow' becomes 'man' (fellow can mean a man).
'ladies'+'man'='LADIES' MAN'
(Other definitions for ladies man that I've seen before include "Womaniser" , "(Would be) seducer of women?" , "Say, Don Giovanni" , "Lothario" , "Libertine" .)