Swift in air (7)
I believe the answer is:
express
I believe this clue is a double definition.
'swift' is the first definition.
(I've seen this before)
'air' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for express that I've seen before include "Utter - non-stop" , "Convey; high-speed" , "special delivery" , "Explicit" , "Put into words - fast" .)