Northern sea duck (5)
I believe the answer is:
eider
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">northern sea duck</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for eider that I've seen before include "Quilt- furnishing duck" , "Breed of duck" , "Duck valued for its fine soft down" , "Source of fine soft down" , "Sea-duck" .)