Bitter feud (8)
I believe the answer is:
vendetta
'bitter feud' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vendetta that I've seen before include "Family blood feud" , "Lasting feud associated with Sicily and mafia" , "A prolonged feud, associated with Sicily" , "Violent feud" , "A bitter Italian feud" .)