Copyright on Greek isle building material (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'copyright on greek isle' is the wordplay.
'copyright' becomes 'C' (as in the copyright symbol, a C in a circle).
'greek isle' becomes 'Crete' (I've seen this in another clue).
'c'+'on'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "It's composed of sand, cement and water" , "factual type of evidence?" , "Substantial, real" , "that may be reinforced" , "Actual - stuff made with cement" .)