Many-headed monster (5)
I believe the answer is:
hydra
'many-headed monster' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hydra that I've seen before include "Stars" , "Many-headed monster of Greek mythology" , "Small red-berried tree" , "Multi-headed monster killed by Hercules" , "Monster Hercules slew" .)