Hawk always returns South (4)
I believe the answer is:
eyas
'hawk' is the definition.
(eyas is a kind of hawk)
'always returns south' is the wordplay.
'always' becomes 'aye' (archaic term for 'always'**).
'returns' says the letters should be written in reverse.
'south' becomes 's' ('S' can be a synonym of 'south').
'aye' back-to-front is 'eya'.
'eya'+'s'='EYAS'
(Other definitions for eyas that I've seen before include "Young bird" , "young hawk" , "Unfledged hawk" , "Nestling hawk" , "Nestling bird" .)