Wistful exhalation (4)
I believe the answer is:
sigh
'wistful exhalation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sigh that I've seen before include "resigned response" , "Audible exhalation of breath" , "Deep breath expressing regret, weariness etc" , "Express dejection" , "Utterance made by exhaling audibly" .)