Agent in favour of entertaining turn (6)
I believe the answer is:
factor
'agent' is the definition.
(business term for a representative)
'in favour of entertaining turn' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'turn' becomes 'act' (theatre).
'for' going around 'act' is 'FACTOR'.
(Other definitions for factor that I've seen before include "Integer that can be exactly divided into another" , "Contributing influence; business agent" , "Contributory element" , "Causal contribution to an outcome" , "Seven is one for thirty five" .)