The most common and most likely answer to this crossword clue is the 7 letter word BECLOUD
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More clues leading to the result BECLOUD
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 = becloud.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 BECLOUD mean?
BECLOUD verb- To cause to become obscure or muddled.
- (usually passive) To cover or surround with clouds.
- To cast in a negative light, cast a pall over, darken.