Carbon and carbonised material? Support monetary incentive (8)
I believe the answer is:
cashback
'monetary incentive' is the definition.
(incentive to buy something)
'carbon and carbonised material? support' is the wordplay.
'carbon' becomes 'C' (chemical symbol).
'and' is a charade indicator (letters next to each other).
'carbonised material?' becomes 'ash' (remains of a fire).
'support' becomes 'back' (to back is to support or endorse).
'c'+'ash'+'back'='CASHBACK'
(Other definitions for cashback that I've seen before include "Debit card facility" , "means to get extra notes" , "Possible bonus" , "supermarket service" , "Way of obtaining money at the till" .)