Short breath (4)
I believe the answer is:
gasp
'short breath' 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 "Breathe irregularly" , "Struggle for breath with open mouth" , "Puff, struggle for breath" , "First Greek letter" , "Catch one's breath in astonishment" .)