Coral reef (5)
I believe the answer is:
atoll
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coral reef</span>' is the definition.<br/ ><span class="explanation-format__explanation">(atoll is a kind of coral reef)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for atoll that I've seen before include "Allot (anag.)" , "Bikini, perhaps" , "Coral isle" , "Pacific coral island" , "land threatened by rising sea levels" .)