Lagoon surrounder (5)
I believe the answer is:
atoll
'lagoon surrounder' 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 "Pacific coral island" , "Allot (anag.)" , "Ring-shaped coral island" , "Circular coral reef" , "Coral circle" .)