Audibly breathe a cunning plan (6)
I believe the answer is:
wheeze
I believe this clue is a double definition.
'audibly breathe' is the first definition.
(breathing while making a wheezing sound)
'a cunning plan' is the second definition.
(I've seen this before)
(Other definitions for wheeze that I've seen before include "A good trick" , "Amusing trick" , "Breathe noisily; trick" , "Clever scheme; sound asthmatic" , "Sound in trouble when articulating" .)