What is best to reduce fat in soup?

You could use a fat separator if you have one, or you could try to do it the old fashioned way with a ladel or a large spoon. But if you have time before you need the broth, there's a much easier way to do it: refrigerate the broth or stock overnight. The fat weighs less than the stock, so it floats up to the surface, turns an off white color and hardens, whereas the stock is a much darker color and usually gelatinizes. So you can just take a spoon and scoop the fat off the top, and can put the gelatinized stock in a saucepan and re-liquify it by warming it up.

I'll often do this with canned broth as well, refrigerate it the day before, then when I open the can the next day, I can just pick up the thin little piece of solidified fat off the top before I use the broth.