Familiar river duck, small, vanishing from Irish county (6)
I believe the answer is:
common
'familiar' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(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" .)