Mythical underworld (5)
I believe the answer is:
hades
'mythical underworld' 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 "Underworld, hell" , "Greek god of the underworld" , "In Greek myth, the Underworld" , "It's classical hell in the shade" , "A place of some darkness" .)