Monetary incentive remains after Conservative support (8)
I believe the answer is:
cashback
'monetary incentive' is the definition.
(incentive to buy something)
'remains after conservative support' is the wordplay.
'remains' becomes 'ash' (remains of a fire).
'after' says to put letters next to each other.
'conservative' becomes 'C' (UK political abbreviation).
'support' becomes 'back' (to back is to support or endorse).
'ash' after 'c' is 'cash'.
'cash'+'back'='CASHBACK'
(Other definitions for cashback that I've seen before include "Debit card facility" , "supermarket service" , "Way of obtaining money at the till" , "Possible bonus" , "means to get extra notes" .)