Exhalation of relief (4)
I believe the answer is:
sigh
'exhalation of relief' 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 "Exhale resignedly" , "resigned response" , "Exhale deeply" , "Exhale loudly" , "Exhalation of breath, with relief say" .)