Pro about to go on stage finds agent (6)
I believe the answer is:
factor
'agent' is the definition.
(business term for a representative)
'pro about to go on stage' is the wordplay.
'pro' becomes 'for' (I've seen this before).
'about' is an insertion indicator.
'to go on stage' becomes 'act' (I can't explain this - if you can you should believe this answer much more).
'for' enclosing 'act' is 'FACTOR'.
'finds' acts as a link.
Can you help me to learn more?
(Other definitions for factor that I've seen before include "Thirteen is one of fifty two" , "Aspect" , "Circumstance that leads to result or mathematical expression" , "Contributory element" , "Level on sun-screen scale" .)