What actors make up in what’s only fair play (6)
I believe the answer is:
equity
This clue is a double definition.
'what actors make up' is the first definition.
(UK trade union for actors)
'what's only fair play' is the second definition.
'in' acts as a link.
(Other definitions for equity that I've seen before include "Value of a property less debt" , "Group of performers" , "Fairness, impartial treatment" , "Fairness for actors' trade union" , "Moral justice for the actor's union" .)