The most common and most likely answer to this crossword clue is the 8 letter word ETHYLENE
I've seen this crossword clue in The Times
E
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Y
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The straight part
We know that "Gas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gas = ethylene.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 ETHYLENE mean?
ETHYLENE noun- The common name for the organic chemical compound ethene. The simplest alkene, a colorless gaseous (at room temperature and pressure) hydrocarbon with the chemical formula C2H4.
- The divalent radical derived from ethane.