Sudden clever idea (5,4)

I think it could be either:

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5,4

brain wave

'sudden clever idea' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for brain wave that I've seen before include "Brilliant notion" , "Sudden good idea" , "A sudden inspiration or bright idea" , "Sudden clever idea" , "Brilliant idea" .)

I've seen this clue in The Mirror.

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