Pond dross (4)
I believe the answer is:
scum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pond dross</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for scum that I've seen before include "Fruit of the oak" , "Floating muck" , "Floating impurities or bad people" , "Impurities floating on surface, or despicable people" , "Worthless group of people" .)