Back Labour against a Conservative plot (5)
I believe the answer is:
cabal
'plot' is the definition.
(I know that cabal is a more specific form of the action plot)
'back labour against a conservative' is the wordplay.
'back' shows that the letters should be reversed in order.
'labour' becomes 'lab' (abbreviation).
'against' means one lot of letters go next to another.
'conservative' becomes 'C' (UK political abbreviation).
'lab'+'a'+'c'='labac'
'labac' backwards is 'CABAL'.
(Other definitions for cabal that I've seen before include "Secret plot or the people engaged in it" , "Group in plot" , "Intrigue, clique" , "Secret political clique or faction" , "Small group involved with intrigue" .)