Study an island made of sand and cement (8)
I believe the answer is:
concrete
'sand and cement' is the definition.
(I've seen this before)
'study an island' is the wordplay.
'study' becomes 'con' (con can informally mean to study).
'an island' becomes 'Crete' (Greek island).
'con'+'crete'='CONCRETE'
'made of' is the link.
(Other definitions for concrete that I've seen before include "Strong, hard, building material" , "Solid like building material" , "Specific; definite" , "Specific - building material" , "Firm" .)