Sea-eagle (4)
I believe the answer is:
erne
'sea-eagle' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for erne that I've seen before include "Sea bird: lough in Ulster" , "White-tailed eagle" , "'Sea-eagle, river and lough (4)'" , "European sea eagle" , "Irish river, loughs; sea bird" .)