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