The most common and most likely answer to this crossword clue is the 5 letter word SURLY
I've seen this crossword clue in The Guardian
S
U
R
L
Y
More clues leading to the result SURLY
Buddy explains!

The straight part
We know that "rude" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rude = surly.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 SURLY mean?
SURLY adjective- Irritated, bad-tempered, unfriendly.
- Threatening, menacing, gloomy.
- Lordly, arrogant, supercilious.
- In an arrogant or supercilious manner.