The most common and most likely answer to this crossword clue is the 6 letter word TWILIT
I've seen this crossword clue in Independent
T
W
I
L
I
T
More clues leading to the result TWILIT
- Fairly dim idiot concealing bad cut
- Somewhat dim fool I left on board
- Fool hems in one inexperienced driver in the half-dark
- Visible in the gloom, clown masks sick for the most part
- Dim person accepting a lot of crime, say, not being bright
- Fool attaining 51 gets gloomy
- After 50, I fool around — that’s how things are at the end of the day
- Silly person crosses one line in the half-dark
Buddy explains!

The straight part
We know that "not being bright" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not being bright = twilit.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 TWILIT mean?
TWILIT verb- To illuminate faintly.
- Illuminated by or as if by twilight.