A rotten crew and they're most impolite (3,7)

I believe the answer is:

bad manners

I'm a little stuck... Click here to teach me more about this clue!

'most impolite' is the definition.
The definition and answer are not the same part of speech.

'a rotten crew and they're' is the wordplay.
'a rotten' becomes 'bad' (I've seen this in another clue).
'crew' becomes 'man' (both can mean to staff or operate something).
'and' means one lot of letters go next to another.
'they're' becomes 'ners' (I am not sure about this - if you are sure you should believe this answer much more).
'bad'+'man'+'ners'='BAD MANNERS'

Can you help me to learn more?

(Another definition for bad manners that I've seen is " breach of etiquette".)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.