Letters sent by plane (3,4)
(Other definitions for air mail that I've seen before include "Overseas postal system" , "Post sent overseas" , "I'm a liar (anag.)" , "Flying post?" , "Post via plane" .)
(Other definitions for airmail that I've seen before include "Flying post?" , "Letters taken by plane" , "Postal service" , "Quick way to deliver" , "communication" .)