Underworld god (5)
I believe the answer is:
hades
'underworld god' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hades that I've seen before include "Mythological abode of the dead" , "Inferno" , "The nether world, hell" , "Greek god of the underworld" , "It's hell in the Greek shade" .)