I don't want to sound harsh, but I don't want to offer false hope. Ducks can make miraculous recoveries, but you need to be aware of some things.
One critical item to take care of is getting the duck into an area with no flies. They will lay eggs in the open wound and add horror to horror and pain to pain.
If bone is exposed, that is a very rough thing. My vet told me that exposed bone can get infected and be impossible to treat.
If bones are not exposed, and you quickly get the duck away from fly risk (think indoors, in a cage with screening around it, here is what Storey's Guide (as I recall) recommends:
get the duck to a clean quiet place away from flies.
clip the feathers from around the wound
put something like sevin powder very very carefully on the skin and feathers around the wound, not in the wound
The Ultimate Pet Duck Guidebook says:
If your duck has any lacerations or bite marks, you are going to want to use a saline solution to do a quick flushing of the wounds. Then use gauze pads to apply direct pressure to stop the bleeding. Once bleeding is controlled, you can lightly spray the injury with a non-sting wound wash.
If you have Baytril on hand, give our duck one 22.7 mg antibiotic pill to help prevent infection.
Get the duck to a vet.
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Addition:
So I would flush the wounded area, clip feathers back at least half an inch away from the edge of the wound, and examine. If I knew a vet or anyone with the ability to help, I would call on them immediately. I would flush the wound with saline solution and put sevin around the outer edges, very carefully. I would put a light coating of triple antibiotic ointment (not pain relief type, it is toxic to ducks) on one side of a gauze pad and hold that in place by wrapping the duck with vet wrap.
I would put the duck in my sick bay in my house, and call a vet.