The most common and most likely answer to this crossword clue is the 9 letter word SOLFATARA
I've seen this crossword clue in Financial Times
S
O
L
F
A
T
A
R
A
Buddy explains!
The straight part
We know that "gas-emitting volcanic vent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gas-emitting volcanic vent = solfatara.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 SOLFATARA mean?
SOLFATARA noun- An area of volcanic activity that gives off sulfurous steam.