What's played a lot and mostly unlike much music? (6)
I believe the answer is:
atonal
'unlike much music?' is the definition.
(most music is tonal rather than atonal)
'what's played a lot and mostly' is the wordplay.
'what's played' indicates anagramming the letters.
'mostly' means to remove the last letter.
'a'+'lot'+'and'='alotand'
'alotand' with its last letter removed is 'alotan'.
'alotan' anagrammed gives 'ATONAL'.
(Other definitions for atonal that I've seen before include "Lacking traditional musical tone structure" , "(Of music) lacking key" , "An alto (anag) - without a fixed key" , "Describes music not written in a particular key" , "Keyless" .)