Really good, which is odd (9)

I believe the answer is:

matchless

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'odd' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for matchless that I've seen before include "Second to none" , "team without a fixture" , "beyond compare" , "Unrivalled" , "Unique" .)

I've seen this clue in the Evening Standard.

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