Mind-boggling (11)

I think it could be either:

Which length description are you looking for?:

astonishing

'mind-boggling' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for astonishing that I've seen before include "Mind-boggling" , "Amazing" , "impressive" , "Greatly surprising" .)

I've seen this clue in The Mirror.

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