Breathe heavily, pant (4)
I believe the answer is:
puff
This clue is a double definition.
'breathe heavily' is the first definition.
(I've seen this in another clue)
'pant' is the second definition.
(I know that pant can be written as puff)
(Other definitions for puff that I've seen before include "Drag on fag; be out of breath" , "type of pastry" , "Short explosive burst of breath" , "out of which pants come?" , "Light pastry; some wind" .)