Big bag in room containing antique? (7)
I believe the answer is:
holdall
'big bag' is the definition.
(I've seen this before)
'room containing antique?' is the wordplay.
'room' becomes 'hall' (hall is a kind of room**).
'containing' means one lot of letters goes inside another.
'antique?' becomes 'old' ('old' can be similar in meaning to 'antique').
'hall' enclosing 'old' is 'HOLDALL'.
'in' acts as a link.
(Other definitions for holdall that I've seen before include "Large bag (no need for another?)" , "Largish bag" , "Capacious bag or basket" , "Handled bag" , "Large carrying bag" .)