Runners do it (4)
I believe the answer is:
pant
'runners do it' 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 "Sound out of breath" , "Breath rapidly" , "Long" , "Gasp for breath (like a hot dog?)" , "Breath noisily from exertion" .)