Fabulous beast (7)
I believe the answer is:
unicorn
'fabulous beast' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unicorn that I've seen before include "Fabulous horned animal" , "Heraldic beast on the Royal Arms" , "Mythical horned equine" , "Team of three" , "Imaginary animal" .)