React with shock (4)
I believe the answer is:
gasp
'react with shock' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gasp that I've seen before include "Convulsive breath" , "Have trouble breathing" , "Pant" , "Breathe sharply in amazement, horror etc" , "Show amazement" .)