The most common and most likely answer to this crossword clue is the 5 letter word DUSKY
I've seen this crossword clue in The Times
D
U
S
K
Y
Buddy explains!

The straight part
We know that "Not bright" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not bright = dusky.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 DUSKY mean?
DUSKY noun- A dusky shark.
- A dusky dolphin.
- A dusky grouse.
- Dimly lit, as at dusk (evening).
- Having a shade of color that is rather dark.
- Dark-skinned.
- Ashen; having a greyish skin coloration.