A bird engaged in making her nest (4)
I believe the answer is:
erne
'a bird' is the definition.
(erne is a type of bird)
'engaged in making her nest' is the wordplay.
'engaged in' indicates a hidden word.
'ERNE' is hidden within 'making her nest'.
(Other definitions for erne that I've seen before include "Eagle, river and lough" , "Fish-eating sea eagle" , "Sea eagle and Northern loch" , "A high-flier" , "European sea eagle" .)