Clean-a-thon - Tradução em português

The -thon suffix means something like "maratona/crusada/gincana", etc... (evento ou competição em favor de alguma causa, arrecadação de fundos, etc.)

There´s is the teleton, radioton, walkathon; readathon, bird-a-thon (maratona de observação de pássaros), cycle-a-thon contest.
All meaning that there is an event, sale, or context involved, with the gist of raising money, to a prearranged goal, or when certain ammount of money is reached. And in this case the "clean-a-thon" (uma maratona de limpeza com fins de arrecadação de fundos para alguma causa.)

PP, o interlocutor provavelmente se referiu ao fato de que teria uma "maratona de limpeza" (uma limpeza monstro mas, ao que me parece, sem fins de arrecadação).

That´s right, Marcio, in this specific case it is. And in many others as well, thanks for pointing that out.
There are cases in which they think about money raising as well, including the ones to the benefit of the cleaning itself.

http://greenplanet4kids.com/blog-entry/ ... fundraiser