Dirty froth or foam (4)
I believe the answer is:
scum
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">dirty froth</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">foam</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that scum is a foam)</span><br/ ><br/ ></span>
(Other definitions for scum that I've seen before include "Offscourings" , "dirty film" , "Refuse" , "to be seen on top" , "Floating impurities on liquid" .)