Scum (5)
I believe the answer is:
dross
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">scum</span>' is the definition.<br/ ><span class="explanation-format__explanation">(dross is a kind of scum)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dross that I've seen before include "Waste from molten metal" , "Rubbish, scum" , "Scum on molten metal" , "Scum on molten metals" , "Something of inferior quality" .)