Coupon (7)
I believe the answer is:
voucher
'coupon' is the definition.
(I know that coupon can be written as voucher)
This is all the clue.
(Other definitions for voucher that I've seen before include "Coupon that can be redeemed" , "Something that can be exchanged for goods or services" , "Ticket" , "Coupon; receipt" , "Document that serves as evidence of some expenditure" .)