Marsupial depositing droppings round old pile of rocks (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

An envelope (‘depositing … round’) of O (‘old’) plus TOR (‘pile of rocks’) in POO (‘droppings’). POO might possibly be a verb, but does not match ‘depositing’. The definition is, I think, rather loose; the potoroo is a genus of Australian animals resembling rodents, but not belonging to the order of rodents.