Conservative-Socialist split (5)
I believe the answer is:
cleft
'split' is the definition.
(I've seen this before)
'conservative-socialist' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'socialist' becomes 'left' ('left' is associated in meaning with 'socialist').
'c'+'left'='CLEFT'
(Other definitions for cleft that I've seen before include "division" , "Split; fissure" , "Fissure; divided" , "Crack" , "Split, divided" .)