A graduate copper's calculating device (6)
I believe the answer is:
abacus
'calculating device' is the definition.
(I've seen this before)
'a graduate copper's' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'copper's' becomes 'Cu's' (chemical symbol).
'a'+'ba'+'cus'='ABACUS'
(Other definitions for abacus that I've seen before include "Counting device" , "Counting machine" , "Old counting-frame" , "Beaded counting frame" , "An old counting frame with beads" .)