Brilliance in Ulster assembly (6)
I believe the answer is:
lustre
'brilliance' is the definition.
(I've seen this before)
'ulster assembly' is the wordplay.
'assembly' is an anagram indicator.
'ulster' anagrammed gives 'LUSTRE'.
'in' acts as a link.
(Other definitions for lustre that I've seen before include "Period" , "Sheen or renown" , "Glory, distinction" , "Gentle sheen/glow" , "Radiance; reflective surface" .)