Worthless matter (5)
I believe the answer is:
dross
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">worthless matter</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 dross that I've seen before include "Rubbish, scum" , "Rubbish; impurities" , "Refuse - dregs" , "Impurity" , "Worthless matter" .)