Ulster manoeuvring shows brilliance (6)
I believe the answer is:
lustre
'brilliance' is the definition.
(I've seen this before)
'ulster manoeuvring' is the wordplay.
'manoeuvring' is an anagram indicator.
'ulster' anagrammed gives 'LUSTRE'.
'shows' is the link.
(Other definitions for lustre that I've seen before include "Gloss - type of pottery" , "Reflected light, sheen" , "Distinction < in five years" , "Sheen or renown" , "Radiance; reflective surface" .)