Coral barrier (4)
I believe the answer is:
reef
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coral barrier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for reef that I've seen before include "Chain of rocks underwater" , "Free (anag.)" , "Underwater obstruction" , "Rocky ridge near sea surface" , "Rock near water's surface" .)