Announce delivery of mail (4)
I believe the answer is:
post
This clue is a triple definition!
'announce' is the first definition.
(I know this)
'delivery' is the second definition.
(I've seen this before)
'of' acts as a link.
'mail' is the third definition.
(I know that mail can be written as post)
(Other definitions for post that I've seen before include "Maybe letters" , "Job; upright timber" , "Stake, job or mail" , "trading establishment" , "employment" .)