Briefly brilliant, shrewd, but just about burnt out (6)

I believe the answer is:

flashy

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'out' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for flashy that I've seen before include "Cheap and showy" , "Gaudy or showy" , "Garish" , "Cheap and ostentatious" , "Ostentatious - glittering" .)

I've seen this clue in the Evening Standard.

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