The most common and most likely answer to this crossword clue is the 6 letter word RADIUM
I've seen this crossword clue in The Times
R
A
D
I
U
M
More clues leading to the result RADIUM
- A police officer taken in by odd element
- An element of air mixed with mud
- Dangerous element runs for street in sports venue
- Fresh air and mud has curative powers
- Air and mud mixture produces 25
- Element which can cause a glow from the Royal Academy
- Metal headbanging primarily avoided by Madhuri while dancing
- Mostly bone and scarcely any muscle
Buddy explains!

The straight part
We know that "Dangerous element" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dangerous element = radium.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 RADIUM mean?
RADIUM noun- The chemical element (symbol Ra) with an atomic number of 88. It is a soft, shiny and silvery radioactive alkaline earth metal.
- A type of cloth woven from silk or synthetic yarn, often with a shiny appearance.
- To treat (a tumour, etc.) with radium.