Sounds like a duck (6)
I believe the answer is:
quacks
'sounds like a duck' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for quacks that I've seen before include "Duck sounds or dud doctors" , "Makes a noise like a duck" , "Charlatan physicians" , "Bad doctors or duck sounds" , "Makes sounds like a duck" .)