The most common and most likely answer to this crossword clue is the 6 letter word MIASMA
I've seen this crossword clue in The Times
M
I
A
S
M
A
More clues leading to the result MIASMA
- Noxious gas is dispersing, after bottling by old lady
- Girl's mother creates oppressive atmosphere
- Bad atmosphere some teams aimed to overturn
- Recalled morning target to limit source of some smell
- Morning’s end sees return of bad atmosphere
- Maternal grandmother inhaling Italy’s noxious air?
- Setter beset by Grandma’s
- An unpleasant smell coming from my respiratory condition, they say.
- Married to bureaucrat, mother raising a stink
- A writing with intent rejected as something obnoxious
Buddy explains!

The straight part
We know that "Noxious gas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case noxious gas = miasma.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 MIASMA mean?
MIASMA noun- A noxious atmosphere or influence.
- A noxious atmosphere or emanation once thought to originate from swamps and waste, and to cause disease.