Love in sin, say (5)
I believe the answer is:
voice
'say' is the definition.
(I know that voice is a more specific form of say)
'love in sin' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'in' is an insertion indicator.
'sin' becomes 'vice'.
'o' put inside 'vice' is 'VOICE'.
(Other definitions for voice that I've seen before include "Sound of speech" , "Express, utter" , "eg Bass" , "Sound produced in the larynx" , "Sound of speaker" .)