If you can get the duck to the vet, that may be best. In the meantime, it looks like a foot wound, indeed, with some infection, and you can treat it. Bumblefoot often has a smaller, dark center, not an open wound like that. It almost appears that someone may have tried to treat the bumblefoot by opening it.
With this problem, instead of pus, there is material that has a texture more like cottage cheese (apologies, but that seems the best description we have come up with). I try to catch it early so we don't need to open the foot, risking further infection.
For this, see if you can let the duckling swim in some clean (we know, it doesn't stay pristine) lukewarm water for a while (at least 15 minutes). Then apply some triple antibiotic ointment. I apply the ointment three times a day, initially (after a quick cleaning or wipe-off of the foot), then after a few days when there is improvement, twice a day, morning and evening (especially evening), for a couple of weeks, then once a day till it is gone. I go case by case to decide how thoroughly to clean, how often to treat, and whether to confine the duck. Usually I don't have to get too carried away, but this one looks troublesome.
Good for you, caring enough to deal with this!
The vet may be able to help you with some oral antibiotics and if this is an older infection, there is a risk it has gone systemic, and the topical ointment won't do the job.