Breathe hard or pant (4)
I believe the answer is:
blow
I believe this clue is a double definition.
'breathe hard' is the first definition.
(I've seen this before)
'pant' is the second definition.
(I know that pant is a more specific form of blow)
(Other definitions for blow that I've seen before include "Sudden disappointment" , "Waste" , "force through pipe" , "Bang, wallop" , "'..., ..., thou winter wind' (As You Like It)" .)