Duck sounds (6)
I believe the answer is:
quacks
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">duck sounds</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 quacks that I've seen before include "Duck sounds or dud doctors" , "bird sounds" , "Makes a noise like a duck" , "Charlatan physicians" , "Ducks' noises" .)