The most common and most likely answer to this crossword clue is the 7 letter word SHADOWY
I've seen this crossword clue in The Guardian
S
H
A
D
O
W
Y
More clues leading to the result SHADOWY
- Mysterious flash around plug
- Curious how day’s dim
- Mysterious flash enveloping trailer
- Vague, pompous commercial breaks
- Flamboyant display took in the obscure
- Originally, Star Wars has Yoda struggling on the dark side?
- Insubstantial commercial set in flashy surroundings
- Notice loud clothes, being dark and mysterious
- Secretive or ostentatious about ad
Buddy explains!

The straight part
We know that "Mysterious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mysterious = shadowy.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 SHADOWY mean?
SHADOWY adjective- In shadow; darkened by shadows.
- (of character) Dark, obscure.
- Indulging in fancies; daydreaming.