Moon of Pluto (6)
I believe the answer is:
charon
'moon of pluto' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for charon that I've seen before include "In Greek myth, ferryman of Hades" , "Ferryman of dead (Gk. myth)" , "He ferries dead over Styx" , "Hell's boatman on the Styx" , "Ferryman on river of Hades" .)