Electoral system is blocking true revenge (8)
I believe the answer is:
reprisal
'revenge' is the definition.
(I know that reprisal is a type of revenge)
'electoral system is blocking true' is the wordplay.
'electoral system' becomes 'pr' (proportional representation).
'blocking' indicates putting letters inside (to block can mean to plug a gap in something).
'true' becomes 'real' (similar in meaning).
'pr'+'is'='pris'
'pris' going inside 'real' is 'REPRISAL'.
(Other definitions for reprisal that I've seen before include "Tit for tat" , "revenge attack" , "Counterattack" , "Act of revenge" , "Retaliatory action against an enemy" .)