Island that's a ring (5)
I believe the answer is:
atoll
'island' is the definition.
(I know that atoll is a type of island)
'a ring' is the wordplay.
'ring' becomes 'toll' (a bell tolls).
'a'+'toll'='ATOLL'
'that's' is the link.
(Other definitions for atoll that I've seen before include "Coral island(s) surrounding a lagoon" , "String of islands" , "Pacific coral island" , "Bikini, perhaps" , "Ring-shaped coral reef enclosing a lagoon" .)