POW camp of a prisoner (5)
I believe the answer is:
oflag
'pow camp' is the definition.
(type of German prisoner of war camp)
'of a prisoner' is the wordplay.
'a prisoner' becomes 'lag' (slang term).
'of'+'lag'='OFLAG'
(Other definitions for oflag that I've seen before include "camp, not for men" , "prison camp" , "Colditz, for example" , "German PoW camp for officers" , "Place of internment" .)