Adult cygnet (4)
I believe the answer is:
swan
'adult cygnet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for swan that I've seen before include "possibly mute" , "Bird of Cygnus genus" , "Large aquatic bird" , "Whooper or trumpeter?" , "black, in 7th" .)