The most common and most likely answer to this crossword clue is the 9 letter word GLITTERED
I've seen this crossword clue in The Times
G
L
I
T
T
E
R
E
D
More clues leading to the result GLITTERED
Buddy explains!

The straight part
We know that "Was brilliant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was brilliant = glittered.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
GLITTERED verb- To sparkle with light; to shine with a brilliant and broken light or showy luster; to gleam.
- To be showy, specious, or striking, and hence attractive.