One representing, for a long time, a theatre? (5)
I believe the answer is:
agent
'one representing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'long time a theatre?' is the wordplay.
'long time' becomes 'age' (an age is a period of time).
'a theatre?' becomes 'NT' (National Theatre).
'age'+'nt'='AGENT'
'for a' is the link.
Can you help me to learn more?
(Other definitions for agent that I've seen before include "envoy" , "Spy - broker" , "Spy; representative" , "Author" , "Performer's manager" .)