Blackboard material (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">blackboard material</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 slate that I've seen before include "Run down" , "A roof-tile; criticise heavily" , "Grey stone; tab" , "list of candidates" , "Blast" .)