Postage sticker (5)
I believe the answer is:
stamp
'postage sticker' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stamp that I've seen before include "Bring foot down hard" , "Impress with a mark" , "What to put on 7 before putting in 15" , "Use a die; character" , "making an impression?" .)