Realm surrounded by the Styx (5)
I believe the answer is:
hades
'realm surrounded by the styx' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hades that I've seen before include "Greek god of the underworld" , "It's hell in the Greek shade" , "Home of the souls of the dead" , "A place of some darkness" , "In Greek myth, the Underworld" .)