Say there's love in sin? (5)
I believe the answer is:
voice
'say' is the definition.
(voicing is a kind of saying)
'love in sin?' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'in' means one lot of letters goes inside another.
'sin?' becomes 'vice'.
'o' placed into 'vice' is 'VOICE'.
'there's' is the link.
(Other definitions for voice that I've seen before include "Sound of speech" , "Computerised analysis of speech" , "tenor, say" , "Spoken sound" , "State" .)