Hostility's accepted if not a sin (8)
I believe the answer is:
enormity
'a sin' is the definition.
(thesaurus)
'hostility's accepted if not' is the wordplay.
'hostility' becomes 'enmity' (synonyms).
'accepted' is an insertion indicator.
'if not' becomes 'or' (both can be said before introducing an alternative).
'enmity' placed around 'or' is 'ENORMITY'.
(Other definitions for enormity that I've seen before include "Outrageous act" , "horror" , "Outrageous deed" , "Outrage" , "Extreme wickedness or vastness" .)