Awful rudeness of a number not wearing clothes (9)

I believe the answer is:

undressed

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'awful rudeness of a number' is the definition.
I can't tell whether this definition defines the answer.

'not wearing clothes' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'wearing clothes' becomes 'dressed' (I've seen this before).
'un'+'dressed'='UNDRESSED'

Can you help me to learn more?

(Other definitions for undressed that I've seen before include "Removed clothing" , "Exposed" , "Disrobed" , "Lacking kit" , "Took the clothes off" .)

I've seen this clue in The Globe and Mail.

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