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

I believe the answer is:

bad manners

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'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' (I've seen this before).
'and' becomes 'n' (common abbreviation for 'and').
'they're' becomes 'ers' (I can't justify this - if you can you should believe this answer much more).
'bad'+'man'+'n'+'ers'='BAD MANNERS'

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

I've seen this clue in the Evening Standard.

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