Take in air (7)
I believe the answer is:
breathe
'take in air' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for breathe that I've seen before include "Take into mouth" , "Draw air into, and expel out of, the lungs" , "Absorb air" , "Take air in and out of the lungs" , "use aqualungs, perhaps" .)