The most common and most likely answer to this crossword clue is the 9 plus 1 letter word IMPROPER} –
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!

The straight part
We know that "Inappropriate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case inappropriate = improper} –.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 IMPROPER} – mean?
IMPROPER} – verb- To appropriate; to limit.
- To behave improperly
- Unsuitable to needs or circumstances; inappropriate; inapt
- Not in keeping with conventional mores or good manners; indecent or immodest
- Not according to facts; inaccurate or erroneous
- Not consistent with established facts; incorrect
- Not properly named; See, for example, improper fraction
- Not specific or appropriate to individuals; general; common.