Most brilliant but least serious (8)
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" .)