Conservative in rage broke the rules (7)
I believe the answer is:
cheated
'broke the rules' is the definition.
(to cheat is to break the rules of a game)
'conservative in rage' is the wordplay.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'in rage' becomes 'heated' (heated can mean angry).
'c'+'heated'='CHEATED'
(Other definitions for cheated that I've seen before include "Defeated through deceit" , "Behaved dishonestly" , "taken for a ride" , "stung" .)