Last word about copper’s penetration (6)
I believe the answer is:
acumen
'penetration' is the definition.
(thesaurus)
'last word about copper's' is the wordplay.
'last word' becomes 'amen' (I've seen this before).
'about' means one lot of letters goes inside another.
'copper' becomes 'cu' (chemical symbol).
'amen' placed around 'cu' is 'ACUMEN'.
(Other definitions for acumen that I've seen before include "Kind of sagacity, good for business" , "Shrewdness, intelligence in business" , "Mental sharpness as in business" , "Good judgment" , "perspicacity" .)