The most common and most likely answer to this crossword clue is the 6 letter word VAPOUR
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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 = vapour.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 VAPOUR mean?
VAPOUR noun- Cloudy diffused matter such as mist, steam or fumes suspended in the air.
- The gaseous state of a substance that is normally a solid or liquid.
- Something insubstantial, fleeting, or transitory; unreal fancy; vain imagination; idle talk; boasting.
- Any medicinal agent designed for administration in the form of inhaled vapour.
- (in the plural) Hypochondria; melancholy; the blues; hysteria, or other nervous disorder.
- Wind; flatulence.
- To become vapor; to be emitted or circulated as vapor.
- To turn into vapor.
- To emit vapor or fumes.
- To use insubstantial language; to boast or bluster.
- To give (someone) the vapors; to depress, to bore.