Opponent behind Conservative prank (5)
I believe the answer is:
antic
'prank' is the definition.
(synonyms)
'opponent behind conservative' is the wordplay.
'opponent' becomes 'anti' (I've seen this before).
'behind' says to put letters next to each other.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'anti'+'c'='ANTIC'
(Other definitions for antic that I've seen before include "Playful or mischievous act" , "seen to be bizarre, at one time" , "Caper, trick" , "Absurd act" , "Mad act done for fun" .)