Conservative member's ascent (5)
I believe the answer is:
climb
'ascent' is the definition.
(I know that ascent can be written as climb)
'conservative member's' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'member' becomes 'limb' (member can mean a limb).
'c'+'limb'='CLIMB'
(Other definitions for climb that I've seen before include "Go higher up" , "improve one's social status" , "increase" , "showing the way up" , "mountaineer's intent?" .)