Never seen that on a lease before. Maintaining the property, yes. But "clean", eh???
Wow, sweeping generalisation there.
Delgirl has the 'clean' clause in her lease. And she's entitled to put it there. But her tenant's have to abide by it. Sounds perfectly reasonable. As I don't have leases I don't impose it. And based on the OP's story, so far nothing has come out that allows an eviction. It is highly likely though that the tenants will use the deposit as last months rent and OP will have to pay someone to massively clean the house. The way they keep the house says a lot about the tenants.
In relation to your sweepign generalistion on cleaning up after tenant's. I'd say about 90% of my tenants who left a property required some kind of cleaning. It might only be the oven and fridge, this seems to be the area that impacts me, but I'm racking my brains to think of tenant's who left a place completely not needing some kind of work. I just write it down as a cost of business. Sometimes have had to buy new cookers such is the state of it. I should have shares in the electrical companies as they get a lot of spending from me. Another one is hoovers, the amount of people unable to operate them is beyond me.