Influence loud performer (6)
I believe the answer is:
factor
'influence' is the definition.
(thesaurus)
'loud performer' is the wordplay.
'loud' becomes 'f' (A musical term).
'performer' becomes 'actor' (actor is a kind of performer).
'f'+'actor'='FACTOR'
(Other definitions for factor that I've seen before include "Scottish estate manager" , "Cause" , "Level on sun-screen scale" , "Causal contribution to an outcome" , "Circumstance that contributes toward a result - arithmetically?" .)