Country producing copper and barium (4)
I believe the answer is:
cuba
'country' is the definition.
(Cuba is an example)
'copper and barium' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'and' means one lot of letters go next to another.
'barium' becomes 'ba' (Ba is the chemical symbol for barium).
'cu'+'ba'='CUBA'
'producing' acts as a link.
(Other definitions for cuba that I've seen before include "Its capital is Havana" , "Country famous for its cigars" , "Largest West Indies island" , "Guantanamo Bay location" , "Caribbean island republic" .)