Common wild duck (7)
I believe the answer is:
mallard
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">common wild 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 mallard that I've seen before include "web-footed bird" , "Type of duck - world speed record holding steam engine (1938)" , "Engine holding the world speed record for a steam locomotive, set at 123mph in 1938" , "The famous steam train" , "Ancestor of all domestic ducks" .)