The most common and most likely answer to this crossword clue is the 11 letter word MALPRACTICE
I've seen this crossword clue in The Times
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More clues leading to the result MALPRACTICE
Buddy explains!

The straight part
We know that "Unethical behaviour/)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unethical behaviour/) = malpractice.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 MALPRACTICE mean?
MALPRACTICE noun- The improper treatment of a patient by a physician that results in injury or loss.
- Improper or unethical conduct by a professional or official person.