Tempered, toughened (8)
I believe the answer is:
annealed
This clue is a double definition.
'tempered' is the first definition.
(similar in meaning)
'toughened' is the second definition.
(annealing is a kind of toughening)
(Other definitions for annealed that I've seen before include "Strengthened" , "made tougher" , "Like heat-treated metal" .)