Copper has last word about shrewdness (6)
I believe the answer is:
acumen
'shrewdness' is the definition.
(acumen can mean shrewdness or good judgment)
'copper has last word about' is the wordplay.
'copper has' becomes 'cu' (I've seen this in other clues).
'last word' becomes 'amen' (last word in a prayer).
'about' means one lot of letters goes inside another.
'cu' inserted inside 'amen' is 'ACUMEN'.
(Other definitions for acumen that I've seen before include "brightness" , "Intellectual sharpness as in business" , "Sagicity as in business" , "Ability to judge and decide quickly" , "Business sense" .)