Crossword Genius

Party acknowledging voting rights? (10)

Ross

I believe the answer is:

eighteenth

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for eighteenth that I've seen is " Last hole in golf".)

I've seen this clue in the Sydney Morning Herald.
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