The most common and most likely answer to this crossword clue is the 9 letter word BRIGHTEST
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result BRIGHTEST
Buddy explains!

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.
What does BRIGHTEST mean?
BRIGHTEST adjective- Visually dazzling; luminous, lucent, clear, radiant; not dark.
- Having a clear, quick intellect; intelligent.
- Vivid, colourful, brilliant.
- Happy, in good spirits.
- Sparkling with wit; lively; vivacious; cheerful.
- Illustrious; glorious.
- Clear; transparent.
- Manifest to the mind, as light is to the eyes; clear; evident; plain.