One note at party is unfriendly (10)
I believe the answer is:
antisocial
'unfriendly' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'one note at party' is the wordplay.
'one note at' becomes 'anti' (I am not sure about this - if you are sure you should believe this answer much more).
'party' becomes 'social' (social can mean a party or event).
'anti'+'social'='ANTISOCIAL'
'is' is the link.
Can you help me to learn more?
(Other definitions for antisocial that I've seen before include "Not getting on with others" , "withdrawn" , "Disruptive to the community" , "Disinclined to mix" , "Reluctant to mix" .)