Large aquatic bird (4)
I believe the answer is:
swan
'large aquatic bird' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for swan that I've seen before include "Large aquatic bird" , "Royal bird" , "Kind of song, in the end" , "Mute, whooper or Bewick's bird" , "The Ugly Duckling's outcome" .)