Common aquatic bird (7)
I believe the answer is:
moorhen
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">common aquatic bird</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for moorhen that I've seen before include "Water bird" , "Female red grouse or waterfowl" , "pond bird" , "Common aquatic bird" , "Black waterbird" .)