Greek sorceress, niece of Circe (5)
I believe the answer is:
medea
I believe this clue is a double definition.
'greek sorceress' is the first definition.
(I've seen this before)
'niece of circe' is the second definition.
(I've seen this in another clue)
(Other definitions for medea that I've seen before include "Jason's sorceress" , "Tragedy by Euripedes" , "Play by Euripides" , "Sorceress involved with Jason" , "Mythical enchantress" .)