Firm's do supported by island (8)
I believe the answer is:
concrete
'firm's' is the definition.
(I've seen this before)
'do supported by island' is the wordplay.
'do' becomes 'con' (both can mean to defraud).
'supported by' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'island' becomes 'crete' (Greek island).
'con'+'crete'='CONCRETE'
(Other definitions for concrete that I've seen before include "Strong, hard, building material" , "Substantial, real" , "Building material is not abstract" , "Unsightly building material" , "Actual - stuff made with cement" .)