Note, one in key, mostly, or not in key (6)
I believe the answer is:
atonal
'not in key' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
'note one in key mostly' is the wordplay.
'note' becomes 'la' (musical note as in do-re-mi).
'one in key' becomes 'tonal' (I've seen this in another clue).
'mostly' means to remove the first letter (1).
'la'+'tonal'='latonal'
'latonal' with its initial letter removed is 'ATONAL'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for atonal that I've seen before include "An alto (anag) - without a fixed key" , "Musically, having no established key" , "Keyless" , "Lacking traditional musical tone structure" , "(Music) having no established key" .)