Not just a mixture of fun and music (6)
I believe the answer is:
unfair
'not just' is the definition.
('unfair' is the opposite to 'just')
'a mixture of fun and music' is the wordplay.
'a mixture of' indicates anagramming the letters (letters mixed up).
'and' is a charade indicator (letters next to each other).
'music' becomes 'air' (air is a kind of music).
'fun' anagrammed gives 'unf'.
'unf'+'air'='UNFAIR'
(Other definitions for unfair that I've seen before include "Swizz!" , "Not just < dark" , "Weighted" , "Partisan" , "Not equitable - or blonde?" .)