A grotesque slur, due to cheating (10)
I believe the answer is:
adulterous
'cheating' is the definition.
(synonyms)
'a grotesque slur due to' is the wordplay.
'grotesque' indicates an anagram.
'slur'+'due'+'to'='slurdueto'
'slurdueto' is an anagram of 'dulterous'.
'a'+'dulterous'='ADULTEROUS'
(Other definitions for adulterous that I've seen before include "untrue" , "Betraying a legal partner" .)