The most common and most likely answer to this crossword clue is the 5 letter word ACIDS
I've seen this crossword clue in The Daily Telegraph
A
C
I
D
S
Buddy explains!

The straight part
We know that "dangerous substances" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dangerous substances = acids.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 ACIDS mean?
ACIDS noun- A sour substance.
- Any of several classes of compound having the following properties:
- Lysergic acid diethylamide (LSD)