The most common and most likely answer to this crossword clue is the 7 letter word STYGIAN
S
T
Y
G
I
A
N
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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 = stygian.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 STYGIAN mean?
STYGIAN adjective- Dark and gloomy.
- Infernal or hellish.