Breathe heavily (4)
'breathe heavily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pant that I've seen before include "Breathe hard" , "Breath rapidly" , "Breath noisily from exertion" , "Long" , "Breathe noisily after running" .)
(Other definitions for puff that I've seen before include "Light cake; type of sleeve" , "Light < case" , "Exaggerated praise (for promotional purposes?)" , "Light pastry; some wind" , "Fruit" .)