Certain island behind fraud (8)
I believe the answer is:
concrete
'certain' is the definition.
(thesaurus)
'island behind fraud' is the wordplay.
'island' becomes 'crete' (Greek island).
'behind' means one lot of letters go next to another.
'fraud' becomes 'con' (con is a kind of fraud).
'crete' put after 'con' is 'CONCRETE'.
(Other definitions for concrete that I've seen before include "Solid, real" , "Building material - tangible" , "Specific - building material" , "Real, not abstract" , "Specific; definite" .)