The most common and most likely answer to this crossword clue is the 7 letter word KRYPTON
K
R
Y
P
T
O
N
Buddy explains!

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 = krypton.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 KRYPTON mean?
KRYPTON noun- The chemical element (symbol Kr) with an atomic number of 36. It is a colourless, odourless noble gas that only reacts with fluorine. It is one of the rarest gases in the Earth's atmosphere.
- An atom of this element.