A sin. That's understood (6)
I believe the answer is:
agreed
'understood' is the definition.
(thesaurus)
'a sin' is the wordplay.
'sin' becomes 'greed' (greed is a kind of sin).
'a'+'greed'='AGREED'
'that's' is the link.
(Other definitions for agreed that I've seen before include "Shared an opinion" , "Saw eye to eye" , "Of one mind" , "Said yes" , "ratified" .)