Crossword Genius

Most brilliant but least serious (8)

Ross

I believe the answer is:

lightest

This clue is a double definition.

'most brilliant' is the first definition.
(brilliant can mean lit or shining)

'least serious' is the second definition.
('light' is the opposite of 'serious' so 'lightest' should be a synonym for 'least serious')

'but' is the link.

(Other definitions for lightest that I've seen before include "Least dark or least heavy" , "Having the least weight" , "With the least 22 ac" .)

I've seen this clue in The Telegraph.
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