The most common and most likely answer to this crossword clue is the 10 letter word PEA-SOUPER
I've seen this crossword clue in The Times
P
E
A
-
S
O
U
P
E
R
More clues leading to the result PEA-SOUPER
- After gym start to overexert in a cool fog
- Veg sounding wonderful, but we see nothing in it?
- Exercise over after being enveloped by a fantastic fog
- What made Londoners choke quietly as Europe disintegrated
- Vegetable said to be very good, but nothing’s visible in this
- Quietly use a rope to get around in thick fog
- Go out of this world, say, in zero visibility
Buddy explains!

The straight part
We know that "fog" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fog = pea-souper.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 PEA-SOUPER mean?
PEA-SOUPER noun- A dense, yellowish fog, often mixed with smoke; a pea-soup fog, a smog.
- A French-Canadian person, especially a Francophone from the province of Québec.