So sorry for your duck! 
I agree with Kathy, vet would be best, if possible... if not, keep it clean and bandaged, padded well to help prevent further damage... isolated and quiet, shock and trauma can be worse than wounds sometimes... watch for signs of infection, excessive heat (remember their body temps are higher than ours, though), redness, swelling, discoloration or any bad smell...
Sling might help her to keep off of it, or at least pillows to make her a channel to lay in...
What breed is she? Heavier weight breeds have more complications adapting to one leg, lighter weight breeds have a higher chance of adapting...
Not all get picked on, and some do quite well with one leg... my Peg is a Call (so much smaller) and she actually hatched with only one leg, but she gets around just as well as her flock mates... she can swim straight and she balances on one leg just fine... only uses her wings to steady herself occasionally...
So don't give up hope, they're tougher and more resilient than we think!

I agree with Kathy, vet would be best, if possible... if not, keep it clean and bandaged, padded well to help prevent further damage... isolated and quiet, shock and trauma can be worse than wounds sometimes... watch for signs of infection, excessive heat (remember their body temps are higher than ours, though), redness, swelling, discoloration or any bad smell...
Sling might help her to keep off of it, or at least pillows to make her a channel to lay in...
What breed is she? Heavier weight breeds have more complications adapting to one leg, lighter weight breeds have a higher chance of adapting...
Not all get picked on, and some do quite well with one leg... my Peg is a Call (so much smaller) and she actually hatched with only one leg, but she gets around just as well as her flock mates... she can swim straight and she balances on one leg just fine... only uses her wings to steady herself occasionally...
So don't give up hope, they're tougher and more resilient than we think!
