Express surprise (4)
I believe the answer is:
gasp
'express surprise' 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 sharply in amazement, horror etc" , "Utter breathlessly" , "Show amazement" , "Noisily breathe" , "Catch one's breath in astonishment" .)