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">(eider is a kind of sea duck)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for eider that I've seen before include "Duck from which down is obtained" , "Duck supplying down" , "Reed I turn into a duck" , "I've noted down" , "Duck with soft feathers" .)