Crossword Genius

Brightness perhaps lacking in flat (6)


I believe the answer is:


'brightness' is the definition.
(lustre is a kind of brightness)

I don't understand how the remainder of the clue works.

(Other definitions for lustre that I've seen before include "Gloss, sheen" , "Polish" , "Rustle up the sheen" , "number of years" , "Gentle sheen/glow" .)

I've seen this clue in The Guardian.
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