The most common and most likely answer to this crossword clue is the 6 letter word ARSINE
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The straight part
We know that "Poisonous gas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case poisonous gas = arsine.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 ARSINE mean?
ARSINE noun- A compound of arsenic and hydrogen, AsH3, a colorless and exceedingly poisonous gas, having an odor like garlic.
- Any organic derivative of this compound, or of diarsane, triarsane etc.