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 "Greek god of darkness, son of Chaos", "Volcanic mountain in Antarctica", "The abode of the dead", "Dark and gloomy cavern between earth and Hades.", "The underworld", "Be sure (anag) - Greek god", "Volcano in Antarctica", "Mt. -, Antarctic volcano; be sure (anag.)", "God of darkness - cavern on the way to Hades - active volcano in Antarctica" and "Darkness".)