Rail: currently large bird (4,3)
I believe the answer is:
barn owl
'bird' is the definition.
(barn owl is a kind of bird)
'rail currently large' is the wordplay.
'rail' becomes 'bar' (I've seen this in other clues).
'currently' becomes 'now' (right now).
'large' becomes 'L' (eg in clothes sizes).
'bar'+'now'+'l'='BARN-OWL'
(Other definitions for barn owl that I've seen before include "Nocturnal raptor" , "Endangered night bird" , "Nocturnal bird native to Britain" , "Flier" , "Large hunting bird" .)