Contrarily, I'm this when rude! (6)

I believe the answer is:

polite

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'rude' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the rest of the clue.

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(Other definitions for polite that I've seen before include "Having good manners, socially correct" , "Respectful and considerate" , "Cultivated" , "Well-spoken and civil" , "(Of society) refined" .)

I've seen this clue in the Evening Standard.

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