Island in the form of 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' (ring a bell).
'a'+'toll'='ATOLL'
'in the form of' acts as a link.
(Other definitions for atoll that I've seen before include "Coral island(s) surrounding a lagoon" , "Land around a lagoon, such as Bikini" , "Pacific coral island" , "tropical paradise?" , "Allot (anag.)" .)