Unhappy about empty theatre being filled (5)
I believe the answer is:
sated
'being filled' is the definition.
(I've seen this in another clue)
'unhappy about empty theatre' is the wordplay.
'unhappy' becomes 'sad' (associated in meaning).
'about' is an insertion indicator.
'empty' means to remove the middle letters.
'theatre' with its middle removed is 'te'.
'sad' placed around 'te' is 'SATED'.
(Other definitions for sated that I've seen before include "fully indulged" , "Stuffed, having eaten to the full" , "Indulged to the full" , "Glutted" , "given more than enough" .)