Agent of a loud Thespian (6)
I believe the answer is:
factor
'agent' is the definition.
(business term for a representative)
'a loud thespian' is the wordplay.
'a loud' becomes 'f' (short for forte - 'loud' in music).
'thespian' becomes 'actor' (synonyms).
'f'+'actor'='FACTOR'
'of' acts as a link.
(Other definitions for factor that I've seen before include "Cause" , "Act for (anag)" , "Reason" , "Mathematical quantity" , "Seven is one for thirty five" .)