You, oldie, go in for an anteroom (5)
I believe the answer is:
foyer
'an anteroom' is the definition.
('foyer' can be a synonym of 'anteroom')
'you oldie go in for' is the wordplay.
'you oldie' becomes 'ye' (I can't explain this - if you can you should believe this answer much more).
'go in' indicates putting letters inside.
'ye' inserted within 'for' is 'FOYER'.
Can you help me to learn more?
(Other definitions for foyer that I've seen before include "Theatrical" , "Theatre's entrance hall" , "Reception hall" , "Theatre entrance hall" , "Lobby of hotel, etc." .)