Overseas post system (3,4)
I believe the answer is:
air mail
'overseas post system' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for air mail that I've seen before include "Overseas post option" , "Letters carried by plane" , "Letters that are flown in" , "I'm a liar (anag.)" , "Overseas post system" .)