The most common and most likely answer to this crossword clue is the 7 letter word VAPOURS
V
A
P
O
U
R
S
Buddy explains!

The straight part
We know that "mists" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mists = vapours.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 VAPOURS mean?
VAPOURS 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.