Arresting copper is the last word in shrewdness (6)
I believe the answer is:
acumen
'shrewdness' is the definition.
(acumen can mean shrewdness or good judgment)
'arresting copper is the last word' is the wordplay.
'arresting' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'is the last word' becomes 'amen' (I've seen this before).
'cu' going into 'amen' is 'ACUMEN'.
'in' is the link.
(Other definitions for acumen that I've seen before include "Discernment - insight" , "Intellectual sharpness as in business" , "Penetrating insight" , "Ability to take well judged decisions" , "The ability to make good and quick decisions" .)