The most common and most likely answer to this crossword clue is the 6 letter word RUDEST
I've seen this crossword clue in The Daily Telegraph
R
U
D
E
S
T
More clues leading to the result RUDEST
Buddy explains!

The straight part
We know that "most ill-mannered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most ill-mannered = rudest.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 RUDEST mean?
RUDEST adjective- Bad-mannered.
- Somewhat obscene, pornographic, offensive.
- Tough, robust.
- Undeveloped, unskilled, basic.
- Hearty, vigorous; found particularly in the phrase rude health.