Coral ridge (4)
I believe the answer is:
reef
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coral ridge</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for reef that I've seen before include "Kind of knot (avoided by sailors?)" , "Ridge of rock underwater" , "The Great Barrier ..., Australian natural wonder" , "Ridge of coral near the sea surface" , "Low-lying coral ridge" .)