The Ugly Duckling, in reality
I believe the answer is:
swan
'the ugly duckling in reality' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for swan that I've seen before include "Mute, whooper or Bewick's bird" , "Flighty" , "Whooper or trumpeter?" , "... Australian river" , "Large water bird" .)