Strong metal (4)
I believe the answer is:
iron
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strong metal</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 iron that I've seen before include "1 down, symbol Fe" , "Box-shaped instrument of torture -- no meridian (anag)" , "Smooth - metal" , "Fe - golf club" , "Metallic element, symbol Fe" .)