Building material for many on a Greek island (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'many on a greek island' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'a greek island' becomes 'crete' (I've seen this before).
'c'+'on'+'crete'='CONCRETE'
'for' is the link.
(Other definitions for concrete that I've seen before include "Building material - tangible" , "Specific - building material" , "Actual - stuff made with cement" , "Solid, real" , "Real; building material" .)