Outside a theatre's interior curtains (5)
I believe the answer is:
death
'curtains' is the definition.
(I've seen this before)
'outside a theatre's interior' is the wordplay.
'interior' indicates a hidden word (I've seen 'interior of' mean this).
'DEATH' is hidden within 'outside a theatres'.
(Other definitions for death that I've seen before include "Demise" , "The end - after 13 across, the plague" , "Last moment of existence" , "killing" , "Cessation of life" .)