Marginal discounts? (7)
I believe the answer is:
rebates
'discounts?' is the definition.
(rebating is a kind of discounting)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rebates that I've seen before include "Monies having to be sent back" , "Refunds of some of the amounts paid" , "enclosure?" , "Retrospective discounts" , "Refunds -- discounts" .)