Unsociable sort is unhappy at party (5)
I believe the answer is:
Here is my best explanation:
'unsociable sort' is the definition.
'unhappy at party' is the subsidiary indication.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy').
'at' is a charade indicator (letters next to each other).
'party' becomes 'do' (do is a kind of party).
'is' is the link.
(Other definitions for saddo that I've seen before include "Nerdy type", "pathetic person", "loser", "Boring loner", "nerdy type", "lonely person" and "He's pathetic!".)