Building material for many on a Mediterranean isle (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'many on a mediterranean isle' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for c00).
'a mediterranean isle' becomes 'crete' (Greek island. I am not sure about the 'mediterranean' bit.).
'c'+'on'+'crete'='CONCRETE'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for concrete that I've seen before include "Real, not abstract" , "Not abstract - construction material" , "Firm" , "Strong, hard, building material" , "setter" .)