Give the dentist an astonished look? (4)

I believe the answer is:

gape

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'look?' is the definition.
(gape is a kind of look)

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

(Other definitions for gape that I've seen before include "Look open-mouthed" , "Gawp" , "Stare at page" , "Look with mouth wide open" , "Open-mouthed stare" .)

I've seen this clue in the Evening Standard.

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