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Effect --> Keying --> Difference Matte
You need to use a technique called rotoscoping to separate the subject from the background. You can then put the subject over the clean plate.
Rotoscoping is not an easy or an automatic technique. You'll find valuable information about it here.
This page has some very good links. You should know that AE's Rotobursh tool can not be used intuitively, and that you must look at the training for it that you'll find in the links in the page above.
I know Rotoscoping and Rotobrushing. I'm looking for an alternative method. You shouldn't have to roto if you already have a clean plate of the background.
You can try Mylenium's Difference Matte suggestion. It might work, depending on the nature of your footage.
But keep a sharp eye on the edges of your subject: if they don't look good to you, they probably will never look great using it. There aren't many ways to tweak a Difference Matte.
If there were easy and accurate techniques to avoid shooting chroma keys, the whole planet would be using them, and chroma keys would be dead. They aren't.