One that's not part of 1, not so unusual (8)

I believe the answer is:

commoner

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'not so unusual' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for commoner that I've seen before include "He's not a member of the nobility" , "not so unusual" , "A non-aristocrat" , "Not a member of the nobility, one of the people" , "more frequently encountered" .)

I've seen this clue in The Guardian.

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