Copper's sphere of operation? (7)
I believe the answer is:
element
I believe this clue is a double definition.
'copper's' is the first definition.
(copper is a chemical element)
'sphere of operation?' is the second definition.
(Other definitions for element that I've seen before include "Basic substance or feature" , "Basic constituent; aspect" , "Constituent part (7)" , "eg Fire, air" , "An abstract part of something" .)