Medusa (9)
I believe the answer is:
jellyfish
'medusa' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for jellyfish that I've seen before include "eg Medusa" , "Marine stinger" , "Gelatinous sea animal with tentacles" , "Sea animal with stinging tentacles" , "Sea creature with stinging tentacles" .)