Jellyfish petrifying woman (6)
I believe the answer is:
medusa
I believe this clue is a double definition.
'jellyfish' is the first definition.
(synonyms)
'petrifying woman' is the second definition.
(woman who petrifies people in Greek myth)
(Other definitions for medusa that I've seen before include "Amused (anag)" , "Gorgon slain by Perseus" , "Gorgon killed by Perseus - jellyfish" , "Mythical woman with snakes for hair" , "Monster" .)