Say nothing in sin (5)
I believe the answer is:
voice
'say' is the definition.
(voicing is a kind of saying)
'nothing in sin' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'in' means one lot of letters goes inside another.
'sin' becomes 'vice'.
'o' inserted inside 'vice' is 'VOICE'.
(Other definitions for voice that I've seen before include "Speak of - mouthpiece" , "Express, utter" , "right to have a say" , "Sound with which one speaks" , "Spokesperson" .)