Post office purchase (5)

I think it could be either:

Which length description are you looking for?:

stamp

'post office purchase' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for stamp that I've seen before include "Impress; postage sticker" , "Bring foot down hard" , "Official mark; bang (foot)" , "contributions?" , "Postage paid label" .)

I've seen this clue in the USA Today.

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