Unsociable sort is unhappy at party (5)
I believe the answer is:
saddo
'unsociable sort' is the definition.
'unhappy at party' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy').
'at' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
'sad'+'do'='SADDO'
'is' is the link.
(Other definitions for saddo that I've seen before include "Boring loner" , "pathetic person" , "loser" , "nerdy type" , "lonely person" .)