One dead on small Ohio island (7)
I believe the answer is:
isolate
'island' is the definition.
(thesaurus)
'one dead on small ohio' is the wordplay.
'one' becomes 'i' (Roman numeral).
'dead' becomes 'late' (similar in meaning).
'on' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'ohio' becomes 'O' (abbreviation for Ohio).
's'+'o'='so'
'late' after 'so' is 'solate'.
'i'+'solate'='ISOLATE'
(Other definitions for isolate that I've seen before include "Keep apart" , "Separate and keep away from other people" , "Maroon" , "Pick out" , "Point out" .)