Mail in a box (4)
I believe the answer is:
ship
'mail in a box' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ship that I've seen before include "Sea-going craft" , "Seagoing boat" , "Transport as on sea-going vessel" , "craft of 1d [ADVENTURE]" , "Vessel that carries passengers or freight" .)