Heat and cool slowly, in order to toughen (6)
I believe the answer is:
anneal
'heat and cool slowly in order to toughen' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for anneal that I've seen before include "Treat (metal, glass) with heat" , "Toughen using heat" , "Heat and cool to temper or toughen" , "Heat and cool slowly in order to toughen" , "Relieve stress by heating" .)