Astonished, when teeth knocked out? (10)

I believe the answer is:

gobsmacked

'astonished' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for gobsmacked that I've seen before include "Flabbergasted" , "bowled over" , "shocked" , "Dumbfounded" , "utterly astonished" .)

I've seen this clue in The Times.

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