The most common and most likely answer to this crossword clue is the 12 letter word ILLEGITIMATE
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More clues leading to the result ILLEGITIMATE
Buddy explains!

The straight part
We know that "improper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case improper = illegitimate.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 ILLEGITIMATE mean?
ILLEGITIMATE noun- A person born to unmarried parents.
- To make illegitimate.
- Not conforming to known principles, or established or accepted rules or standards.
- Not in accordance with the law.
- Not sanctioned by marriage.
- Not correctly deduced.
- Not authorized by good usage; not genuine.
- Involving the fertilization of pistils by stamens not of their own length, in heterogonously dimorphic and trimorphic flowers.