Mythical beast (6)
I believe the answer is:
dragon
'mythical beast' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dragon that I've seen before include "Mythical beast or lizard" , "Creature of Teutonic mythology" , "Fire-breathing, winged mythical monster" , "Flying monster of Welsh legend" , "Mythical fire-breather" .)