Sign of shock (4)
I believe the answer is:
gasp
'sign of shock' 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 "Breathe irregularly" , "Gaps in struggle for breath" , "Draw breath sharply" , "Inhale sharply with astonishment" , "Utter breathlessly" .)