Collieries (5)
I believe the answer is:
mines
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">collieries</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 mines that I've seen before include "Collieries" , "Quarries" , "Pits" .)