Prank by one opposing Conservative (5)
I believe the answer is:
antic
'prank' is the definition.
(I know that prank can be written as antic)
'by one opposing conservative' is the wordplay.
'by' means one lot of letters go next to another.
'one opposing' becomes 'anti' (prefix meaning 'opposing').
'conservative' becomes 'C' (UK political abbreviation).
'anti' next to 'c' is 'ANTIC'.
(Other definitions for antic that I've seen before include "Grotesque, fantastic (archaic)" , "Caper, trick" , "Ludicrous act done for fun" , "Absurd act" , "Mad act done for fun" .)