Had a look round prison, getting unknown drug (4,5)
I believe the answer is:
nose candy
'drug' is the definition.
(slang term for cocaine)
'had a look round prison getting unknown' is the wordplay.
'had a look' becomes 'nosed' (to nose around is to look around).
'round' means one lot of letters goes inside another.
'prison' becomes 'can' (the can is American slang for prison).
'getting' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'nosed' going around 'can' is 'nosecand'.
'nosecand'+'y'='NOSE CANDY'
(Another definition for nose candy that I've seen is " coke in US".)