Rate no sin bad, being a heretic (9)
I believe the answer is:
nestorian
'a heretic' is the definition.
(I've seen this before)
'rate no sin bad' is the wordplay.
'bad' indicates anagramming the letters.
'rate'+'no'+'sin'='ratenosin'
'ratenosin' anagrammed gives 'NESTORIAN'.
'being' acts as a link.
(Other definitions for nestorian that I've seen before include "of Iraqi church" , "patriarch's teaching" , "Heretic" .)