Victim in genuine denial (8)
I believe the answer is:
rebuttal
'denial' is the definition.
(thesaurus)
'victim in genuine' is the wordplay.
'victim' becomes 'butt' (butt is a kind of victim).
'in' is an insertion indicator.
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'butt' put into 'real' is 'REBUTTAL'.
(Other definitions for rebuttal that I've seen before include "Counter-argument" , "Confutation" , "reply to accusation" , "A contradiction of evidence" , "contrary argument" .)