Bittern, egret, swallows, eagle (4)
I believe the answer is:
erne
'eagle' is the definition.
(I know that erne is a type of sea eagle)
'bittern egret swallows' is the wordplay.
'swallows' indicates the answer is hidden within the clue (I've seen 'swallowing' mean this).
'ERNE' is hidden within 'bittern egret'.
(Other definitions for erne that I've seen before include "flyer in the main" , "Marine flyer" , "A high-flier" , "'Sea-eagle, river and lough (4)'" , "one that catches fish?" .)