The effect is an unusual lustre (6)
I believe the answer is:
result
'the effect' is the definition.
('result' can be a synonym of 'effect')
'an unusual lustre' is the wordplay.
'an unusual' indicates anagramming the letters.
'lustre' with letters rearranged gives 'RESULT'.
'is' acts as a link.
(Other definitions for result that I've seen before include "Verdict" , "Ulster (anag) - outcome" , "'Outcome, upshot (6)'" , "Consequence or effect" , "Rustle (anag.)" .)