Many-headed monster of myth (5)
I believe the answer is:
hydra
'many-headed monster of myth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hydra that I've seen before include "Many-headed serpent" , "Mythical nine-headed monster" , "that regenerated after cuts at the top" , "Multi-headed monster slain by Hercules" , "Many-headed foe of Hercules" .)