The most common and most likely answer to this crossword clue is the 9 letter word PLUTONIUM
I've seen this crossword clue in The Times
P
L
U
T
O
N
I
U
M
Buddy explains!
The straight part
We know that "that’s dangerously unstable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s dangerously unstable = plutonium.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 PLUTONIUM mean?
PLUTONIUM noun- A sanctuary dedicated to the Ancient Greek and Roman god Pluto, usually at a location producing poisonous emissions, believed to represent an entrance to the underworld.
- The transuranic chemical element with atomic number 94 and symbol Pu: a silvery-gray radioactive actinide metal that tarnishes when exposed to air.