Sticker on an envelope (5)
I believe the answer is:
stamp
'sticker on an envelope' 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 "What to put on 7 before putting in 15" , "Postal token" , "Mark of approval" , "Item for philatelist" , "Postal label" .)