Case of senior overcome by auditorium (7)
I believe the answer is:
holdall
'case' is the definition.
'senior overcome by auditorium' is the wordplay.
'senior' becomes 'old' ('old' can be similar in meaning to 'senior').
'overcome by' means one lot of letters goes inside another.
'auditorium' becomes 'hall' (I've seen this before).
'old' going within 'hall' is 'HOLDALL'.
'of' acts as a link.
(Other definitions for holdall that I've seen before include "Item of luggage" , "Large strong bag with handles or shoulder strap" , "A large soft travelling bag - stuff everything in" , "Capacious bag or basket" , "Largish bag" .)