Where prisoners eat CO (9)
I believe the answer is:
constable
'where' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'prisoners eat co' is the wordplay.
'prisoners' becomes 'cons'.
'eat co' becomes 'table' (I can't justify this - if you can you should give a lot more credence to this answer).
'cons'+'table'='CONSTABLE'
(Other definitions for constable that I've seen before include "See 1" , "in blue?" , "English landscape painter for a policeman" , "Law enforcer" , "Policeman - landscape artist" .)