One representing, for a long time, the National 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 the national theatre?' is the wordplay.
'long time' becomes 'age' (an age is a period of time).
'the national theatre?' becomes 'NT' (abbreviation).
'age'+'nt'='AGENT'
'for a' acts as a link.
Can you help me to learn more?
(Other definitions for agent that I've seen before include "who can act?" , "Spy - broker" , "envoy" , "Salesman" , "Factor, spy" .)