Dump all the feed out of the feeder into something else, so you can sort out the clog.
Then decide if the feed is fit to be put back in. If it seems dry and clean, you can put it back. If the feed is wet and sticky, it will just gum up again soon. You may be able to give sticky feed to the ducklings in a different style of feeder, if it has not started to spoil yet.
For where to dump the feed: a plastic storage bin, a clean trash can, a bit bowl, even a big plastic bag. Or you can dump the feed in a pile on a sheet or tarp or even an empty tabletop. Basically, you can use anything that will keep track of the feed without getting it wet or dirty.
Once you have it fixed, do keep an eye on the situation, in case it gums up again, so you can try to figure out what is causing this and stop it happening in future.