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" .)