Duck's fancy crest hiding egg (6)
I believe the answer is:
scoter
'duck's' is the definition.
(scoter is a kind of duck)
'fancy crest hiding egg' is the wordplay.
'fancy' indicates anagramming the letters.
'hiding' means one lot of letters goes inside another.
'egg' becomes 'o' (I've seen this in other clues).
'crest' with letters rearranged gives 'scter'.
'scter' going around 'o' is 'SCOTER'.
(Other definitions for scoter that I've seen before include "Species of diving duck" , "Sea diver" , "Large duck -- corset (anag)" , "Sea duck" , "Diving duck with black plumage" .)