Toughen by heating and cooling (6)
I believe the answer is:
temper
'toughen by heating and cooling' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for temper that I've seen before include "Fit of anger - don't lose it" , "Mood, maybe angry" , "Volatile anger - don't lose it" , "Irritability" , "Lose it and lose control" .)