Running to open-air theatre event (9)
I believe the answer is:
operation
'theatre event' is the definition.
(event that takes place in an operating theatre)
'running to open-air' is the wordplay.
'running' is an anagram indicator.
'to'+'open'+'air'='toopenair'
'toopenair' anagrammed gives 'OPERATION'.
(Other definitions for operation that I've seen before include "Handling" , "running" , "In hospital procedure" , "military undertaking" , "Manoeuvre - military action" .)