Prophetess of Greek myth (9)
I believe the answer is:
'prophetess of greek myth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cassandra that I've seen before include "Prophetess of doom" , "Prophetess during the Trojan War whose predictions were true but never believed" , "was unbelievable" , "One who phrophesies disaster, named after ancient Greek woman" , "Prophesying daughter of Priam of Troy" .)