Before, public transport could be hell (6)
I believe the answer is:
Here is my best explanation:
'hell' is the definition.
'before public transport' is the subsidiary indication.
'before' becomes 'ere' (usually literary).
'public transport' becomes 'bus' (bus is a kind of public transport).
'could be' is the link.
(Other definitions for erebus that I've seen before include "Be sure (anag) - Greek god", "Volcanic mountain in Antarctica", "Volcano in Antarctica", "Greek god of darkness, son of Chaos", "Mt. -, Antarctic volcano; be sure (anag.)", "Sure, be in the underworld", "God of darkness - cavern on the way to Hades - active volcano in Antarctica", "Cavern between earth and Hades", "Antarctic volcano; region between earth and Hades" and "Level of the underworld".)