Sharp Conservative stopping benefit (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'conservative stopping benefit' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'benefit' becomes 'aid' (I've seen this in another clue).
'c' inserted into 'aid' is 'ACID'.
(Other definitions for acid that I've seen before include "Sarcastic - tart" , "Alkali's opposite" , "Corrosive substance" , "Morose" , "- - -drop, sweet" .)