Looked in astonishment at the dentist? (5)

I believe the answer is:

gaped

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'looked' is the definition.
(gaping is a kind of looking)

I don't understand how the remainder of the clue works.

(Other definitions for gaped that I've seen before include "Looked stunned" , "Opened mouth wide" , "Stared open-mouthed - yawned" , "shown astonishment" , "Was yawning" .)

I've seen this clue in the Evening Standard.

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