It could be cast, cast initially on an island (8)
I believe the answer is:
concrete
'it could be cast' is the definition.
The definition suggests a singular noun which matches the answer.
'cast initially on an island' is the wordplay.
'initially' indicates taking the first letters.
'an island' becomes 'crete' (Crete is an example).
The first letter of 'cast' is 'c'.
'c'+'on'+'crete'='CONCRETE'
Can you help me to learn more?
(Other definitions for concrete that I've seen before include "Actual - stuff made with cement" , "Building material - tangible" , "Solid, real" , "Real; building material" , "setter" .)