The most common and most likely answer to this crossword clue is the 5 letter word PRUDE
P
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The straight part
We know that "One that is proper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one that is proper = prude.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 PRUDE mean?
PRUDE noun- A person who is or tries to be excessively proper, especially one who is easily offended by matters of a sexual nature.
- Prudish.