Penny, eg (4)
'penny eg' is the definition.
(penny is a kind of coin)
This is all the clue.
(Other definitions for coin that I've seen before include "little money" , "Mint" , "Metal piece of money" , "Money; devise (a word)" , "Metal currency" .)
(Other definitions for copper that I've seen before include "Atomic number 29" , "An element; small coin" , "bluebottle" , "Ductile metal" , "Low-value coin" .)