Temper, as metal (6)
I believe the answer is:
anneal
'temper as metal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)