Breathe quickly (4)
I believe the answer is:
pant
'breathe quickly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pant that I've seen before include "Breathe heavily" , "Gasp for breath (like a hot dog?)" , "Breath noisily from exertion" , "Breathe hard" , "Struggle for breath" .)