Classical theatre Oswald's outside a very long time (5)
I believe the answer is:
odeon
'classical theatre' is the definition.
(I've seen this before)
'oswald's outside a very long time' is the wordplay.
'outside' means to remove the middle letters.
'a very long time' becomes 'eon' (an extremely long period of time).
'oswald' with its middle removed is 'od'.
'od'+'eon'='ODEON'
(Other definitions for odeon that I've seen before include "old theatre" , "Large UK cinema chain" , "Ancient theatre - modern cinema" , "Greek music theatre" , "Ancient Greek theatre" .)