Underworld before Underground alternative (6)
I believe the answer is:
erebus
'underworld' is the definition.
(erebus greek deity born of chaos)
'before underground alternative' is the wordplay.
'before' becomes 'ere' (usually literary).
'underground alternative' becomes 'bus' (london underground tube or bus).
'ere'+'bus'='EREBUS'
(Other definitions for erebus that I've seen before include "Level of the underworld" , "The abode of the dead" , "Mt. -, Antarctic volcano; be sure (anag.)" , "Antarctic volcano; region between earth and Hades" , "God of darkness - cavern on the way to Hades - active volcano in Antarctica" .)