It's not just a mixture of fun and music (6)
I believe the answer is:
unfair
'it's not just' is the definition.
('unfair' is the opposite to 'just')
'a mixture of fun and music' is the wordplay.
'a mixture of' indicates an anagram (I've seen this in other clues).
'and' means one lot of letters go next to another.
'music' becomes 'air' (air is a kind of music).
'fun' with letters rearranged gives 'unf'.
'unf'+'air'='UNFAIR'
(Other definitions for unfair that I've seen before include "Not equitable or even-handed" , "deceitful" , "Dirty" , "not right" , "Swizz!" .)