Stare with an open mouth (4)
I believe the answer is:
gape
'stare with an open mouth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gape that I've seen before include "Failure to meet" , "Open-mouthed stare" , "Stare stupidly" , "Hang wide open" , "Look open-mouthed" .)