Impersonate party animal no longer (4)
I believe the answer is:
dodo
'animal no longer' is the definition.
(extinct animal)
'impersonate party' is the wordplay.
'impersonate' becomes 'do' ('do' can mean to do an impression of).
'party' becomes 'do' (do is a kind of party).
'do'+'do'='DODO'
(Other definitions for dodo that I've seen before include "one finished?" , "Idiot" , "One deeply reactionary" , "Proverbially extinct bird" , "bird long dead" .)