A loud “Not just an amusement park” (7)
I believe the answer is:
funfair
'an amusement park' is the definition.
(I know that amusement park can be written as funfair)
'a loud not just' is the wordplay.
'a loud' becomes 'f' (short for forte - 'loud' in music).
'not just' becomes 'unfair' ('unfair' is the opposite to 'just').
'f'+'unfair'='FUNFAIR'
(Other definitions for funfair that I've seen before include "entertainment for the family?" , "place of amusement" , "Swings and roundabouts" , "Amusements here" , "Amusement park" .)