Unopened post bag (4)
I believe the answer is:
take
'bag' is the definition.
(thesaurus)
'unopened post' is the wordplay.
'unopened' means to remove the first letter.
'post' becomes 'stake' ('stake' can be a synonym of 'post').
'stake' with its initial letter taken away is 'TAKE'.
(Other definitions for take that I've seen before include "Remove; consume" , "Remove; capture" , "Teak (anag.)" , "Capture, secure" , "receipts" .)