The most common and most likely answer to this crossword clue is the 4 letter word WAYS
I've seen this crossword clue in The Guardian
W
A
Y
S
More clues leading to the result WAYS
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 = ways.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 WAYS mean?
WAYS noun- (heading) To do with a place or places.
- A method or manner of doing something; a mannerism.
- A state or condition
- (heading) Personal interaction.
- A tradition within the modern pagan faith of Heathenry, dedication to a specific deity or craft, Way of wyrd, Way of runes, Way of Thor etc.
- Speed, progress, momentum.
- A degree, an amount, a sense.
- (As the head of an interjectory clause, followed by an infinitive starting with “to”) Acknowledges that a task has been done well, chiefly in expressions of sarcastic congratulation.
- The timbers of shipyard stocks that slope into the water and along which a ship or large boat is launched.
- The longitudinal guiding surfaces on the bed of a planer, lathe, etc. along which a table or carriage moves.
- The letter for the w sound in Pitman shorthand.
- (usually preceded by a) A distance.