Bird at two parties (4)
I believe the answer is:
dodo
'bird' is the definition.
(dodo is a kind of bird)
'at two parties' is the wordplay.
'two parties' means two substitutions for 'party'.
'at' is a charade indicator (letters next to each other).
'party' becomes 'do' (do can informally mean a party).
'party' becomes 'do' (do can informally mean a party).
'do'+'do' is 'DODO'.
(Other definitions for dodo that I've seen before include "Square" , "silly person" , "Native of Mauritius" , "Old fogey" , "one very dead!" .)