Monster slain by Hercules (5)
I believe the answer is:
hydra
'monster slain by hercules' 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 "Multi-headed monster slain by Hercules" , "Nine-headed monster killed by Hercules" , "Many-headed snake slain by Hercules" , "Monster Hercules slew" , "Femme fatale of legend" .)