Isle, a state (4)
I believe the answer is:
iowa
'state' is the definition.
(I know that Iowa is an American state)
'isle a' is the wordplay.
'isle' becomes 'iow' (abbreviation for Isle of Wight).
'iow'+'a'='IOWA'
(Other definitions for iowa that I've seen before include "Midwest US state" , "American state, capital Des Moines" , "part of United States" , "US state, capital Des Moines" , "State of USA" .)