The most common and most likely answer to this crossword clue is the 9 letter word IMPRUDENT
I've seen this crossword clue in The Sunday Telegraph
I
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More clues leading to the result IMPRUDENT
- One not heartless describing member with unpleasant rash
- Naughty child, bad-mannered not heartlessly reckless
- . Rash of insulting books on Independent politician
- Indiscreet book comes out after Trump/Biden chaos
- I end in trouble with 22, getting rash
- Reckless rogue with dirty books
- Devil insulting religious text, playing with fire
- Unwise being saucy embracing Romeo
- Audacious banking rules originally, not wise
- I am priggish type, not unwise
Buddy explains!

The straight part
We know that "not wise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not wise = imprudent.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 IMPRUDENT mean?
IMPRUDENT adjective- Not prudent; wanting in prudence or discretion; indiscreet; injudicious; not attentive to consequence; improper.