Greek princess who helped Jason obtain the Golden Fleece (5)
I believe the answer is:
medea
'greek princess who helped jason obtain the golden fleece' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for medea that I've seen before include "Greek princess who helped Jason get the Golden Fleece" , "Mrs Jason" , "Tragic heroine" , "Mythical enchantress" , "Tragedy by Euripedes" .)