Fabulous beast (7)
I believe the answer is:
unicorn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fabulous beast</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)