Islanders preserve excellent metal containers (9)
I believe the answer is:
jamaicans
'islanders' is the definition.
(I have seen 'Caribbean islanders' mean 'jamaicans' so perhaps 'islanders' could also mean 'jamaicans')
'preserve excellent metal containers' is the wordplay.
'preserve' becomes 'jam' (jam is a kind of preserve).
'excellent' becomes 'ai'.
'metal containers' becomes 'cans' (I've seen this before).
'jam'+'ai'+'cans'='JAMAICANS'
(Other definitions for jamaicans that I've seen before include "Caribbean islanders" , "West Indians" .)