Copper back to find something to chew on (3)
I believe the answer is:
cud
'something to chew on' is the definition.
(as in chewing the cud)
'copper back to find' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'back to' indicates one should take the final letters.
The final letter of 'find' is 'd'.
'cu'+'d'='CUD'
(Other definitions for cud that I've seen before include "It's partly digested and chewed again" , "Ruminant animals chew it" , "THIRD PART OF 23" , "Wad of something chewable" , "It's chewed by a ruminant" .)