Birds that have gone with party animals (6)
I believe the answer is:
dodoes
'birds that have gone' is the definition.
(extinct birds)
'with party animals' is the wordplay.
'with' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
'animals' becomes 'does' (doe is a kind of animal**).
'do' put next to 'does' is 'DODOES'.
(Other definitions for dodoes that I've seen before include "Old-fashioned types" , "people with little intelligence" .)