Proof that you’re given satisfaction (7)

I believe the answer is:

receipt

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'proof' is the definition.
(I have seen 'Proof of purchase' mean 'receipt' so perhaps 'proof' could also mean 'receipt')

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for receipt that I've seen before include "Formally acknowledge" , "old-fashioned instructions for cook" , "voucher" , "given for payment" , "Sales slip" .)

I've seen this clue in the Evening Standard.

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