Keen Conservative coming in to help (4)
I believe the answer is:
acid
'keen' is the definition.
(I've seen this before)
'conservative coming in to help' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'coming in to' indicates putting letters inside.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'c' put inside 'aid' is 'ACID'.
(Other definitions for acid that I've seen before include "Sour, sharp-tasting" , "Bitter - drug" , "scathing" , "Caustic and biting" , "Cutting" .)