Temper, as metal (6)
I believe the answer is:
anneal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">temper as metal</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for anneal that I've seen before include "Make tough" , "Toughen by heating and slow cooling" , "Treat (metal, glass) with heat" , "get tough" , "toughening process" .)