The most common and most likely answer to this crossword clue is the 12 letter word IDIOSYNCRASY
I've seen this crossword clue in The Times
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More clues leading to the result IDIOSYNCRASY
Buddy explains!

The straight part
We know that "Peculiarity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case peculiarity = idiosyncrasy.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 IDIOSYNCRASY mean?
IDIOSYNCRASY noun- A behavior or way of thinking that is characteristic of a person.
- A language or behaviour that is particular to an individual or group.
- A peculiar individual reaction to a generally innocuous substance or factor.
- A peculiarity that serves to distinguish or identify.