Building material produced by first company on island (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'by first company on island' is the wordplay.
'by' means one lot of letters go next to another.
'first' suggests taking the first letters.
'island' becomes 'crete' (Greek island).
The initial letter of 'company' is 'c'.
'c' next to 'on' is 'con'.
'con'+'crete'='CONCRETE'
'produced' acts as a link.
(Other definitions for concrete that I've seen before include "Not abstract or imaginary" , "Solid, real" , "Substantial, real" , "It's composed of sand, cement and water" , "Specific; definite" .)