Betrayed astonishment (5)
I believe the answer is:
gaped
'betrayed astonishment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gaped that I've seen before include "Stared open-mouthed - yawned" , "Was yawning" , "shown astonishment" , "Looked stunned" , "Opened mouth wide" .)