Rips off in authentic vengeful act (8)
I believe the answer is:
reprisal
'vengeful act' is the definition.
'rips off in authentic' is the wordplay.
'off' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'authentic' becomes 'real' (I've seen this before).
'rips' with letters rearranged gives 'pris'.
'pris' going within 'real' is 'REPRISAL'.
(Other definitions for reprisal that I've seen before include "revenge attack" , "Act of revenge" , "Retaliatory deed" , "Tit for tat" , "Retaliatory action against an enemy" .)