Dirty froth (4)
I believe the answer is:
scum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dirty froth</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 scum that I've seen before include "Offscourings" , "Floating dirt or froth" , "Impurities on surface of liquid - or bad people" , "Impurities floating on surface, or despicable people" , "Floating film of filth" .)