Rip-off boy in gaol (6)
I believe the answer is:
prison
'gaol' is the definition.
(thesaurus)
'rip-off boy' is the wordplay.
'off' indicates anagramming the letters.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'rip' anagrammed gives 'pri'.
'pri'+'son'='PRISON'
'in' acts as a link.
(Other definitions for prison that I've seen before include "Nick" , "cage" , "Correctional institution" , "Clink - jug" , "Place of incarceration" .)