Speak of love in sin (5)
I believe the answer is:
voice
'speak' is the definition.
(to voice something is to say it)
'love in sin' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'in' is an insertion indicator.
'sin' becomes 'vice'.
'o' inserted into 'vice' is 'VOICE'.
'of' acts as a link.
(Other definitions for voice that I've seen before include "Power of speaking" , "eg Bass" , "Ability to speak, sing" , "Spokesperson" , "Speak of - mouthpiece" .)