The most common and most likely answer to this crossword clue is the 9 letter word CHOKEDAMP
I've seen this crossword clue in The Times
C
H
O
K
E
D
A
M
P
Buddy explains!
The straight part
We know that "unpleasant atmosphere" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unpleasant atmosphere = chokedamp.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.
Crossword Answer definitions
CHOKEDAMP noun- A damp consisting chiefly of carbon dioxide, so called for its ability to asphyxiate.