Either nest hides a bird (4)
I believe the answer is:
erne
'a bird' is the definition.
(erne is a kind of bird)
'either nest hides' is the wordplay.
'hides' indicates a hidden word.
'ERNE' is hidden within 'either nest'.
(Other definitions for erne that I've seen before include "Large bird" , "Seabird" , "European sea eagle" , "A high-flier" , "'Sea-eagle, river and lough (4)'" .)