Egyptian god of the underworld (6)
I believe the answer is:
osiris
'egyptian god of the underworld' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for osiris that I've seen before include "Husband of Isis, father of Horus - sis or I (anag)" , "once highly esteemed by Egyptians" , "dead judge?" , "one held to be divine" , "Egyptian god - sis or I (anag)" .)