Circular coral reef (5)
I believe the answer is:
atoll
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">circular coral reef</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for atoll that I've seen before include "tropical paradise?" , "tropical island" , "Bikini, perhaps" , "Coral isle" , "Ring-shaped coral reef enclosing a lagoon" .)