Postman reported? (4)
I believe the answer is:
'postman' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I can't explain the remainder of the clue.
(Other definitions for mail that I've seen before include "Letters - armour" , "American post from Lima" , "Armour; posted items" , "Dispatch - armour" , "Send message" .)