Breathe with difficulty (6)
I believe the answer is:
wheeze
'breathe with difficulty' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for wheeze that I've seen before include "Breathe noisily; scheme" , "Breathe with hoarse whistling sound" , "A heavy breath or jolly jape" , "Clever scheme or noisy breath" , "Audible inspiration" .)