Copper lead ultimately is something to chew on (3)
I believe the answer is:
cud
'something to chew on' is the definition.
(as in chewing the cud)
'copper lead ultimately' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'ultimately' suggests the final letters.
The last letter of 'lead' is 'd'.
'cu'+'d'='CUD'
'is' is the link.
(Other definitions for cud that I've seen before include "It's partly digested and chewed again" , "Ruminant animals chew it" , "Ruminants chew it (3)" , "Something to be chewed" , "it's still in the cow" .)