Harden or soften (6)
I believe the answer is:
This clue is a double definition.
'harden' is the first definition.
('temper' can be a synonym of 'harden')
'soften' is the second definition.
'or' is the link.
(Other definitions for temper that I've seen before include "Toughen by gradually heating and cooling" , "Adjust" , "Mood or hardness of metal" , "Tendency to become angry easily" , "Harden with heat and water" .)