Getting proof of purchase (7)
I believe the answer is:
receipt
I believe this clue is a double definition.
'getting' is the first definition.
(I know that receipt is a type of getting)
'proof of purchase' is the second definition.
(I've seen this before)
(Other definitions for receipt that I've seen before include "Acceptance" , "A written acknowledgement; a word used in the first ever 2 dn" , "old-fashioned instructions for cook" , "old cook's directions" , "Proof of payment made" .)