The most common and most likely answer to this crossword clue is the 8 letter word QUIETEST
I've seen this crossword clue in Financial Times
Q
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Buddy explains!

The straight part
We know that "It being most peaceful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it being most peaceful = quietest.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 QUIETEST mean?
QUIETEST adjective- With little or no sound; free of disturbing noise.
- Having little motion or activity; calm.
- Not busy, of low quantity.
- Not talking much or not talking loudly; reserved.
- Not showy; undemonstrative.
- Requiring little or no interaction.