Is this one you are hand-raising, or with mom? If you clean the area really well a couple times a day (not sure what's available to you, but antibiotic gel/cream without painkillers in it would be great to put on after you clean the area with betadyne or something similar) and keep the area it's living in super clean and free of poo, it should heal up fine on it's own. You just have to be very careful about bacteria getting in there before this is closed up. It should heal up and get smaller over the next couple days. 
 
Wait, is this the umbilical area of a newly hatched duckling, or the cloaca (where they poo from)? If this is the cloaca, then it has a prolapse and requires very different treatment. I assumed it was the umbilicus at first glance, but now I'm doubting what I'm seeing. Can you confirm?