A sin that's allowed (6)
I believe the answer is:
agreed
'allowed' is the definition.
(to agree to something is to allow it)
'a sin' is the wordplay.
'sin' becomes 'greed' (greed is a kind of sin).
'a'+'greed'='AGREED'
'that's' acts as a link.
(Other definitions for agreed that I've seen before include "acceptable" , "Was fitting" , "That's right" , "matched" , "Gave consent" .)