The most common and most likely answer to this crossword clue is the 5 plus 4 letter word MOUNT ETNA
I've seen this crossword clue in Financial Times
M
O
U
N
T
E
T
N
A
More clues leading to the result MOUNT ETNA
- . Smoker returned an offensive letter from Athens after a short time
- Fixed stake pulled over by horse, a volatile thing
- In Sicily it’s highly dangerous recklessly to name leader of traffickers controlling global organisation
- Meant to unwind? It's a fuming place
- Met aunt on unstable volcano
- One erupting in tournament, agitated after losing rook
Buddy explains!
The straight part
We know that "a volatile thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a volatile thing = mount etna.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.