Street criminal outside a prison (6)
I believe the answer is:
stalag
'prison' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I don't see?
'street criminal outside a' is the wordplay.
'street' becomes 'st' (abbreviation).
'criminal' becomes 'lag' (slang term for a prisoner).
'outside' indicates putting letters inside (some letters go outside others).
'st'+'lag'='stlag'
'stlag' placed around 'a' is 'STALAG'.
Can you help me to learn more?
(Other definitions for stalag that I've seen before include "German World War two prison camp" , "German prison camp in WW2" , "Water at the mouth" , "German prison_camp" , "Type of German prison camp" .)