A graduate copper's calculator (6)
I believe the answer is:
abacus
'calculator' is the definition.
(abacus is a kind of calculator)
'a graduate copper's' is the wordplay.
'graduate' becomes 'ba'.
'copper's' becomes 'cus' (chemical symbol).
'a'+'ba'+'cus'='ABACUS'
(Other definitions for abacus that I've seen before include "Mechanical calculator" , "An old counting frame with beads" , "frame of sorts" , "Simple calculating device" , "that one can count on" .)