Agent for a noisy Thespian (6)
I believe the answer is:
factor
'agent' is the definition.
(business term for a representative)
'a noisy thespian' is the wordplay.
'a noisy' becomes 'f'.
'thespian' becomes 'actor' ('actor' can be a synonym of 'thespian').
'f'+'actor'='FACTOR'
'for' is the link.
(Other definitions for factor that I've seen before include "Contributing element" , "Component, element" , "Seven is one for thirty five" , "Reason" , "Number by which another is exactly divisible" .)