Single sound of bell's ring, in the main (5)
I believe the answer is:
atoll
'ring in the main' is the definition.
(ring-shaped reef - the main can mean the sea)
'single sound of bell's' is the wordplay.
'single' becomes 'a'.
'sound of bell' becomes 'toll' (a bell tolls).
'a'+'toll'='ATOLL'
(Other definitions for atoll that I've seen before include "land threatened by rising sea levels" , "String of islands" , "Circular coral reef" , "Bikini for one" , "tropical paradise?" .)