Revenge of agent, gentleman turning up to get gangster? (8)
I believe the answer is:
reprisal
'revenge' is the definition.
(reprisal is a kind of revenge)
'agent gentleman turning up to get gangster?' is the wordplay.
'agent' becomes 'rep' (short for representative).
'gentleman' becomes 'sir' (I've seen this before).
'turning up' says the letters should be written in reverse.
'to get' says to put letters next to each other.
'gangster?' becomes 'al' (Al Capone).
'sir' back-to-front is 'ris'.
'rep'+'ris'+'al'='REPRISAL'
'of' acts as a link.
(Other definitions for reprisal that I've seen before include "Retaliatory deed" , "revenge attack" , "Retaliation for an injury" , "Retaliatory action against an enemy" , "Counterattack" .)