Roofing stone (5)
I believe the answer is:
slate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">roofing stone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for slate that I've seen before include "rock used in building" , "Scold; hard rock" , "Rubbish" , "Blast" , "A roof-tile; criticise heavily" .)