Study on Mediterranean island building material (8)
I believe the answer is:
concrete
'building material' is the definition.
(concrete is a kind of building material)
'study on mediterranean island' is the wordplay.
'study' becomes 'con' (I've seen this before).
'on' says to put letters next to each other.
'mediterranean island' becomes 'crete' (I've seen this before).
'con'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "that may be reinforced" , "Solid, real" , "Not abstract - construction material" , "Not abstract or imaginary" , "Specific - building material" .)