Building material — cement, originally, on Greek island (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'cement originally on greek island' is the wordplay.
'originally' suggests taking the first letters.
'greek island' becomes 'crete' (I've seen this before).
The initial letter of 'cement' is 'c'.
'c'+'on'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "Unsightly building material" , "Specific; definite" , "Substantial, real" , "Solid, real" , "Building material - tangible" .)