Sea-duck (5)
I believe the answer is:
eider
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">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 "Type of duck" , "Breed of duck" , "Down-producing duck" , "Quilt- furnishing duck" , "Source of fine soft down" .)