Very clever person

I think it could be either:

Which length description are you looking for?:

brainbox

'very clever person' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for brainbox that I've seen before include "Skull - clever clogs" , "Mastermind" , "Clever type" , "Very clever figure" , "quiz champion?" .)

I've seen this clue in The Mirror.

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