Potter's medium (4)
I believe the answer is:
clay
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">potter's medium</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 clay that I've seen before include "Brick/pottery substance" , "slip" , "Basis of pottery" , "Material used for bricks, ceramics etc" , "Earthenware" .)