Building material business on northern Greek Island (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'business on northern greek island' is the wordplay.
'business' becomes 'co' (short for company).
'on' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'greek island' becomes 'crete' (I've seen this before).
'co'+'n'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "Unsightly building material" , "Real, not abstract" , "Substantial, real" , "Firm" , "Actual - stuff made with cement" .)