Study island's building material (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'study island's' is the wordplay.
'study' becomes 'con' (con can informally mean to study).
'island' becomes 'crete' (Greek island).
'con'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "It's composed of sand, cement and water" , "Definite; building material" , "Specific - building material" , "Solid like building material" , "Real; building material" .)