Insult succeeded with brilliance (6)
I believe the answer is:
slight
'insult' is the definition.
(I've seen this before)
'succeeded with brilliance' is the wordplay.
'succeeded' becomes 's' (genealogical abbreviation).
'with' says to put letters next to each other.
'brilliance' becomes 'light' (brilliant can mean shining or lit up).
's'+'light'='SLIGHT'
(Other definitions for slight that I've seen before include "Insult, affront" , "Minor -- snub" , "Flimsy, tenuous" , "Superficial" , "Deliberate discourteous act" .)