A southern bird to the rear (6)
I believe the answer is:
astern
'the rear' is the definition.
(I have seen 'rear of ship' mean 'astern' so perhaps 'rear' could also mean 'astern')
'a southern bird' is the wordplay.
'southern' becomes 's' (abbreviation).
'bird' becomes 'tern' (tern is a kind of bird).
'a'+'s'+'tern'='ASTERN'
'to' acts as a link.
(Other definitions for astern that I've seen before include "Toward the tail of a vessel" , "Towards a ship's rear" , "Nautically behind" , "Toward the back of a boat" , "State of S central US" .)