Atoll, small island (4)
I believe the answer is:
isle
I believe this is a double definition.
'atoll' is the first definition.
(I've seen this in another clue)
'small island' is the second definition.
(I've seen this in another clue)
(Other definitions for isle that I've seen before include "Hawaii, perhaps" , "Such as Man or Bute" , "Eg, eyot or inch" , "I shall hear" , "Christmas or Easter, perhaps" .)