Prisoner hears about United States (9)
I believe the answer is:
countries
'states' is the definition.
('country' can be a synonym of 'state')
'prisoner hears about united' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'hears' becomes 'tries' (to try or hear in the legal sense).
'about' is an insertion indicator.
'united' becomes 'u'.
'con'+'tries'='contries'
'contries' placed around 'u' is 'COUNTRIES'.
(Other definitions for countries that I've seen before include "Cretinous sort of states" , "Bodies of people under single governments" , "States, nations" .)