if it's "webby" you may be able to seperate the backing from the wonderunder after it's zigzagged. do a test first.
after you have the test piece appliqued to the test background, flip it over. try inserting a long pin or needle between the backing and the wonder under. if you can do that without poking through the front, you can use the pin/needle to gently seperate the layers and then clip away the excess backing.
i know this works with stitchwitchery because i've done it. have never tried it with wonderunder, though, so you will have to be the guinea pig for the rest of us. lucky you! :mrgreen: