Leader of the Argonauts who sailed in quest of the Golden Fleece (5)
I believe the answer is:
jason
'leader of the argonauts who sailed in quest of the golden fleece' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for jason that I've seen before include "Argonaut leader" , "Leader of the Argonauts who sailed in quest of the Golden Fleece" , "He led the Argonauts to find the Golden Fleece" , "Chief Argonaut" , "Greek hero, leader of the Argonauts" .)