The most common and most likely answer to this crossword clue is the 10 letter word NAUGHTIEST
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The straight part
We know that "most misbehaving" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most misbehaving = naughtiest.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 NAUGHTIEST mean?
NAUGHTIEST adjective- Mischievous; tending to misbehave or act badly (especially of a child).
- Sexually provocative; now in weakened sense, risqué, cheeky.
- Evil, wicked, morally reprehensible.
- Bad, worthless, substandard.