Birds party and party on, son (5)
I believe the answer is:
dodos
'birds' is the definition.
(I have seen 'Extinct birds ' mean 'dodos' so perhaps 'birds' could also mean 'dodos')
'party and party on son' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'and' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
'on' means one lot of letters go next to another.
'son' becomes 's' (genealogical abbreviation for son).
'do'+'do'+'s'='DODOS'
(Other definitions for dodos that I've seen before include "They never got up" , "Extinct flightless birds" , "Extinct birds of Mauritius" , "they're no more" .)