Startled sound (4)
I believe the answer is:
gasp
'startled sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gasp that I've seen before include "Sound of astonishment" , "First Greek letter" , "Sudden intake of breath" , "Inhale sharply with astonishment" , "Draw breath sharply" .)