Struggle for breath (4)
I believe the answer is:
gasp
'struggle for breath' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gasp that I've seen before include "Short laboured intake of breath" , "Catch one's breath in astonishment" , "Fight for breath" , "Puff, struggle for breath" , "Short sharp intake of breath" .)