Working to create a large base for theatre event (11)
I believe the answer is:
Here is my best explanation:
'working' is the definition.
'a large base for theatre event' is the subsidiary indication.
'large' becomes 'l'.
'base for' is a charade indicator (letters next to each other) (some letters act as a base underneath others).
'theatre event' becomes 'operation' (event that takes place in an operating theatre).
'al' put after 'operation' is 'operational'.
'to create' is the link.
(Other definitions for operational that I've seen before include "In working order", "Ready for use", "In use", "Ready for action" and "Working".)