A ring round a lagoon (5)
I believe the answer is:
atoll
'round a lagoon' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'a ring' is the wordplay.
'ring' becomes 'toll' (a bell tolls).
'a'+'toll'='ATOLL'
Can you help me to learn more?
(Other definitions for atoll that I've seen before include "Key" , "String of islands" , "Ring-shaped coral island" , "Coral circle" , "Ring-shaped coral reef enclosing a lagoon" .)