Acknowledgment of payment (7)
I believe the answer is:
receipt
'acknowledgment of payment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for receipt that I've seen before include "Proof of purchase" , "old cook's directions" , "A written acknowledgement; a word used in the first ever 2 dn" , "Perhaps slip" , "Evidence of sale" .)