Theatre occupied vacant time (5)
I believe the answer is:
odeon
'theatre' is the definition.
(in ancient Greece)
'occupied vacant time' is the wordplay.
'vacant' means to remove the middle letters.
'time' becomes 'eon' (eon is a kind of time**).
'occupied' with its middle removed is 'od'.
'od'+'eon'='ODEON'
(Other definitions for odeon that I've seen before include "dramas here" , "Classical Greek concert hall" , "Greek music hall" , "Ancient Greek concert hall" , "Do one (anag)" .)