Greek god of darkness, son of Chaos (6)
I believe the answer is:
erebus
I believe this is a double definition.
'greek god of darkness' is the first definition.
(I've seen this before)
'son of chaos' is the second definition.
(I've seen this in another clue)
(Other definitions for erebus that I've seen before include "Antarctic volcano; region between earth and Hades" , "In place of darkness" , "Be sure (anag) - Greek god" , "Dark and gloomy cavern between earth and Hades." , "God of darkness - cavern on the way to Hades - active volcano in Antarctica" .)