Inside job? (8,9)
(Other definitions for interior decorator that I've seen before include "Home designer" , "Decor designer" .)
'inside job?' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'inside job?' is the wordplay.
'inside' becomes 'prison' (being 'inside' is being in prison).
'job?' becomes 'officer' (I am not sure about this - if you are sure you should believe this answer much more).
'prison'+'officer'='PRISON OFFICER'
Can you help me to learn more?
(Another definition for prison officer that I've seen is " Warder".)
(Another definition for interior designer that I've seen is " One decorating living spaces".)