"Alas!" sound (4)
I believe the answer is:
sigh
'alas sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sigh that I've seen before include "Sad exhalation" , "Express relief or sorrow" , "Exhale audibly as expression of regret, relief etc" , "Sound of regret" , "Exhale loudly" .)