Ugly Duckling, eventually (4)
I believe the answer is:
'ugly duckling eventually' 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" , "Long-necked water bird" , "The Ugly Duckling's outcome" , "Whooper, perhaps" , "Large, web-footed bird" .)