Definite; building material (8)
I believe the answer is:
concrete
This clue is a double definition.
'definite' is the first definition.
(I've seen this before)
'building material' is the second definition.
(concrete is a kind of building material)
(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" .)