Underwater ridge (4)
I believe the answer is:
reef
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underwater ridge</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 reef that I've seen before include "Coral-ridge" , "endangered habitat" , "Take in (part of sail)" , "Rock in shallow water" , "Rocky ridge near sea surface" .)