Mythical multiheaded beast (5)
I believe the answer is:
hydra
'mythical multiheaded beast' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hydra that I've seen before include "that regenerated after cuts at the top" , "Mythic monster with nine heads" , "Greek island" , "Mythological water-monster" , "Proliferating-head monster" .)