Copper prohibits islanders (6)
I believe the answer is:
cubans
'islanders' is the definition.
(people from Cuba)
'copper prohibits' is the wordplay.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'prohibits' becomes 'bans' (banning is a kind of prohibiting).
'cu'+'bans'='CUBANS'
(Other definitions for cubans that I've seen before include "Nationality of citizens of Havana" , "They're native to Havana, say" , "islanders" , "Nationality of Havana's citizens" , "They live on the largest island in the West Indies" .)