Illicit payment (8)
I believe the answer is:
kickback
'illicit payment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for kickback that I've seen before include "Recoil; illicit payment" , "strong reaction" , "Underhand payment" , "(Illicit) confidential payment to facilitator" , "Bribe - recoil" .)