Reduce copper temperature (3)
I believe the answer is:
cut
'reduce' is the definition.
(I know that reduce can be written as cut)
'copper temperature' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'temperature' becomes 't' (abbreviation).
'cu'+'t'='CUT'
(Other definitions for cut that I've seen before include "Shorten; (one's) share" , "Lower" , "Qualification mark during a golf event" , "Saw" , "Hacked off" .)