Ready to support service for card users (8)
I believe the answer is:
cashback
'service for card users' is the definition.
(bank card service)
'ready to support' is the wordplay.
'ready' becomes 'cash' (ready can informally mean cash).
'to support' becomes 'back' (to back is to support or endorse).
'cash'+'back'='CASHBACK'
(Other definitions for cashback that I've seen before include "supermarket service" , "Debit card facility" , "Possible bonus" , "means to get extra notes" , "Way of obtaining money at the till" .)