Not copper, though it may well contain some (4)
I believe the answer is:
lead
This clue could be a double definition.
'not copper' is the first definition.
'lead' can be an answer for 'copper' (thesaurus). I am unsure of the 'not' bit.
'it may well contain some' is the second definition.
The definition suggests a singular noun which matches the answer.
'though' acts as a link.
Can you help me to learn more?
(Other definitions for lead that I've seen before include "Be in charge of, element" , "Show the way; soft metallic element" , "Church windows include this" , "Guide the heavy metal" , "Metal - van - go ahead" .)