Fight for air (4)
I believe the answer is:
gasp
'fight for air' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gasp that I've seen before include "Inhale sharply with astonishment" , "Noisily breathe" , "Sound of astonishment" , "Struggle for breath with open mouth" , "Say "ah"" .)