The most common and most likely answer to this crossword clue is the 10 letter word RESPIRATOR
R
E
S
P
I
R
A
T
O
R
Buddy explains!
The straight part
We know that "gas mask" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gas mask = respirator.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 RESPIRATOR mean?
RESPIRATOR noun- A device designed to allow breathing when it would otherwise be hindered, as by a medical condition or the presence of poisonous vapors.