Diving duck
I think it could be either:
'diving duck' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eider that I've seen before include "Duck providing fine down" , "Downy sea duck" , "Duck known for down" , "Down comes from this" , "Down-producing duck" .)
'diving duck' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for scaup that I've seen is " Diving duck".)