The most common and most likely answer to this crossword clue is the 8 letter word CHURLISH
I've seen this crossword clue in The Times
C
H
U
R
L
I
S
H
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 = churlish.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 CHURLISH mean?
CHURLISH adjective- Of or pertaining to a serf, peasant, or rustic.
- Rude, surly, ungracious.
- Stingy or grudging.
- (of soil) Difficult to till, lacking pliancy; unmanageable.