Agent in favour of including a stage turn (6)
I believe the answer is:
factor
'agent' is the definition.
(business term for a representative)
'in favour of including a stage turn' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'including' means one lot of letters goes inside another.
'a stage turn' becomes 'act' (I can't explain this - if you can you should believe this answer much more).
'for' enclosing 'act' is 'FACTOR'.
Can you help me to learn more?
(Other definitions for factor that I've seen before include "Circumstance that contributes towards a result" , "one to be taken into account" , "Circumstances" , "Causal contribution to an outcome" , "Act for (anag)" .)