Definite; building material (8)
I believe the answer is:
concrete
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">definite</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">building material</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(concrete is a kind of building material)</span><br/ ><br/ ></span>
(Other definitions for concrete that I've seen before include "It's composed of sand, cement and water" , "Definite; building material" , "Building material - tangible" , "Real; building material" , "Real, not abstract" .)