Coupon one confirms as valid (7)
I believe the answer is:
voucher
I believe this clue is a double definition.
'coupon' is the first definition.
(I know that coupon can be written as voucher)
'one confirms as valid' is the second definition.
(someone who vouches for something)
(Other definitions for voucher that I've seen before include "Document that serves as evidence of some expenditure" , "Coupon that can be redeemed" , "Ticket" , "Document that can be exchanged for goods" , "Coupon; receipt" .)