The most common and most likely answer to this crossword clue is the 9 letter word ETIQUETTE
I've seen this crossword clue in The Guardian
E
T
I
Q
U
E
T
T
E
More clues leading to the result ETIQUETTE
- Courtesy in middle of missive is quite extraordinary
- English mostly snigger about that French behavioural code
- Tchaikovsky’s Sixth loses its way, dry eastern form
- Stripped-off character admits quite bad manners
- Standards of behaviour quite possibly better after extremists leave
- Form one question in English exam, dropping second half of another
- Quite cunning compiler inwardly follows protocol
- Alien, quite altered alien back in good form
- Quite upset by unrestrained character’s manners
- Tweet quite out of order — women ignored acceptable behaviour
Buddy explains!

The straight part
We know that "manners" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case manners = etiquette.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 ETIQUETTE mean?
ETIQUETTE noun- The forms required by a good upbringing, or prescribed by authority, to be observed in social or official life; observance of the proprieties of rank and occasion; conventional decorum; ceremonial code of polite society.
- The customary behavior of members of a profession, business, law, or sports team towards each other.
- A label used to indicate that a letter is to be sent by airmail.