Theatre ruined then (7)
I believe the answer is:
thereat
'then' is the definition.
(thesaurus)
'theatre ruined' is the wordplay.
'ruined' is an anagram indicator.
'theatre' anagrammed gives 'THEREAT'.
(Other definitions for thereat that I've seen before include "On account of that" , "At that place" , "At that point" , "Theatre (anag)" .)