Iron alloy (5)
I believe the answer is:
steel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">iron alloy</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 steel that I've seen before include "Hard, strong, grey alloy" , "brace" , "Knife-sharpener" , "Make hard and unfeeling" , "Mixture of carbon and iron" .)