Reacted in shock (6)
I believe the answer is:
gasped
'reacted in shock' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gasped that I've seen before include "Took a sharp breath" , "Breathed noisily" , "Drew in breath sharply, as in surprise" , "Struggled for breath" , "Drew breath in sharply as in astonishment" .)