Electoral system is stopping existing tit for tat (8)
I believe the answer is:
reprisal
'tit for tat' is the definition.
(I've seen this before)
'electoral system is stopping existing' is the wordplay.
'electoral system' becomes 'pr' (I've seen this in other clues).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'existing' becomes 'real' ('real' can be similar in meaning to 'existing').
'pr'+'is'='pris'
'pris' inserted within 'real' is 'REPRISAL'.
(Other definitions for reprisal that I've seen before include "Act of retaliation or vengeance" , "revenge attack" , "Tit for tat" , "Retaliation for an injury" , "Counterattack" .)