A sin? Yes! (6)
I believe the answer is:
agreed
'yes' is the definition.
(I've seen this before)
'a sin?' is the wordplay.
'sin?' becomes 'greed' (greed is a kind of sin).
'a'+'greed'='AGREED'
(Other definitions for agreed that I've seen before include "Done!" , "Did concur" , "In accord" , "granted" , "Gave consent" .)