Yawned (5)
I believe the answer is:
gaped
'yawned' is the definition.
('gape' can be a synonym of 'yawn')
This is all the clue.
(Other definitions for gaped that I've seen before include "shown astonishment" , "Opened mouth wide" , "Stared open-mouthed - yawned" , "Was yawning" , "Looked stunned" .)