Brightness appearing as clouds started to disperse at intervals (6)
I believe the answer is:
'brightness' is the definition.
(lustre is a kind of brightness)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lustre that I've seen before include "Gloss - type of pottery" , "Sheen - glaze" , "Brilliancy" , "effect of reflected light" , "Distinction < in five years" .)