Reef material (5)
I believe the answer is:
coral
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">reef material</span>' is the definition.<br/ ><span class="explanation-format__explanation">(material making up a coral reef)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for coral that I've seen before include "Basis of reefs" , "Tiny animal forming reef" , "Marine substance, often forming reefs" , "Basic material of South Sea islands" , "Hard substance on the seabed" .)