The most common and most likely answer to this crossword clue is the 8 letter word ALARMING
I've seen this crossword clue in The Sunday Telegraph
A
L
A
R
M
I
N
G
More clues leading to the result ALARMING
- Almost everyone having weapon at home good? Dreadful! (8
- . Marginal difference causing concern
- Marginal changes causing consternation
- Scary gangster providing weapons
- Scary, a leader in London providing weapons
- Dangerous king enters in dynastic style?
- Dreadful, getting tooled up to support the French uprising
- Dreadful, in the manner of king of an ancient dynasty
- Scary weapon encapsulated in a way of speaking almost
- Preparing to fight outside US city — that’s scary
Buddy explains!

The straight part
We know that "Dangerous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dangerous = alarming.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 ALARMING mean?
ALARMING verb- To call to arms for defense
- To give (someone) notice of approaching danger
- To rouse to vigilance and action; to put on the alert.
- To surprise with apprehension of danger; to fill with anxiety in regard to threatening evil; to excite with sudden fear.
- To keep in excitement; to disturb.
- Causing apprehension, fear or alarm; frightening