Short of a vet visit, I really don't know other than supportive care and keeping it contained where it doesn't have to use it's leg till it begins to heal. No guarantee there won't be problems not being set though, sorry this has happened, More than likely if you leave it with the rest it will get stepped on and be in pain.
I have not had to take a duck to the vet, most likely they will want to do an xray to determine where it's broken and how bad, then you'll have decision to make on whether you want to proceed with having it fixed which may mean surgery, but if they feel it can't be fixed or you can't afford it they can put it to sleep for you humanely which would be a good thing .so the duckling won't be suffering. .I would love to tell you everything will be fine and it won't be expensive but I can't. You'll have to decide what you want to do, But either way I don't think I'd keep him with the rest of the ducklings with a broken leg. Hoping the best.
