Player's representative a long time at the theatre (5)
I believe the answer is:
agent
'player's representative' is the definition.
(I've seen this in another clue)
'a long time at the theatre' is the wordplay.
'a long time' becomes 'age' (an age is a period of time).
'at' is a charade indicator (letters next to each other).
'the theatre' becomes 'NT' (National Theatre).
'age'+'nt'='AGENT'
(Other definitions for agent that I've seen before include "Salesman" , "He or she acts for actors" , "Traveller" , "Bond, e.g." , "who can act?" .)