Brighten when you solve the mystery (5,2)

I believe the answer is:

clear up

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'brighten' is the definition.
('clear up' can be a synonym of 'brighten')

I can't explain the rest of the clue.

(Other definitions for clear up that I've seen before include "Straighten out" , "(Of bad weather) improve" , "iron out" , "how conditions may improve" , "Sort out" .)

I've seen this clue in the Evening Standard.

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