Figures out an egghead as being a rascal! (5)

I believe the answer is:

devil

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'rascal' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for devil that I've seen before include "Demon" , "Wicked person" , "Unsurpassed" , "Imp" , "Fiend" .)

I've seen this clue in the Evening Standard.

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