Duck is about to depart from the east (5)

I believe the answer is:

eider

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'duck' is the definition.
(eider is a type of duck)

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

(Other definitions for eider that I've seen before include "Reed I make into a duck" , "Sea duck with soft down" , "Source of fine soft down" , "Duck known for down" , "Large sea duck" .)

I've seen this clue in The Irish Times.

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