Crossword Genius

Familiar river duck, small, vanishing from Irish county (6)

Ross

I believe the answer is:

common

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'familiar' is the definition.
(synonyms)

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

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(Other definitions for common that I've seen before include "Ordinary; green space" , "Shared ground" , "Ten a penny" , "Vulgar; open land" , "Familiar or not refined" .)

I've seen this clue in The Independent.
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