Room containing ancient bag (7)
I believe the answer is:
holdall
'bag' is the definition.
(holdall is a kind of bag)
'room containing ancient' is the wordplay.
'room' becomes 'hall' (hall is a kind of room).
'containing' indicates putting letters inside.
'ancient' becomes 'old' (similar in meaning).
'hall' enclosing 'old' is 'HOLDALL'.
(Other definitions for holdall that I've seen before include "Large strong bag with handles or shoulder strap" , "Handled bag" , "Big bag" , "Capacious bag or basket" , "Largish bag" .)