The most common and most likely answer to this crossword clue is the 7 letter word SUBSIDE
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More clues leading to the result SUBSIDE
Buddy explains!

The straight part
We know that "settle down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case settle down = subside.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 SUBSIDE mean?
SUBSIDE verb- To sink or fall to the bottom; to settle, as lees.
- To fall downward; to become lower; to descend; to sink.
- To fall into a state of calm; to be calm again; to settle down; to become tranquil; to abate.
- To cease talking.