Coral reef island (5)
I believe the answer is:
atoll
'coral reef island' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atoll that I've seen before include "Coral isle" , "Coral circle" , "Bikini, perhaps" , "Circular coral reef" , "Land around a lagoon, such as Bikini" .)