Last word about copper’s insight (6)
I believe the answer is:
acumen
'insight' is the definition.
(I've seen this before)
'last word about copper's' is the wordplay.
'last word' becomes 'amen' (last word in a prayer).
'about' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'amen' placed around 'cu' is 'ACUMEN'.
(Other definitions for acumen that I've seen before include "Awareness" , "Quick intelligence" , "'Keenness, depth of perception (6)'" , "Keen perception" , "Gumption" .)