Struggle to breathe (4)
I believe the answer is:
gasp
'struggle to breathe' 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 "Sudden intake of breath" , "Say "ah"" , "Draw breath sharply" , "Catch one's breath in astonishment" , "Pant" .)