The most common and most likely answer to this crossword clue is the 6 letter word SULLEN
I've seen this crossword clue in The Guardian
S
U
L
L
E
N
Buddy explains!

The straight part
We know that "Dark" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark = sullen.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 SULLEN mean?
SULLEN noun- One who is solitary, or lives alone; a hermit.
- (chiefly in plural) Sullen feelings or manners; sulks; moroseness.
- Having a brooding ill temper; sulky.
- Gloomy; dismal; foreboding.
- Sluggish; slow.
- Lonely; solitary; desolate.
- Mischievous; malignant; unpropitious.
- Obstinate; intractable.