Yes, it's a sin (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'
'it's' acts as a link.
(Other definitions for agreed that I've seen before include "Geared (anag.)" , "Did concur" , "Let's do it!" , "In harmony" , "very well" .)