I am currently modelling a part that, at its present state, has an under cut. I have inserted the image to help me explain what i am trying to achieve.

As you may be able to tell, the shape was an extrusion where two parralel sides where angled. I am intending to have the top edge (which is the smallest) open. Currently this means that the part has an undercut. I have tried increasing the radii, but as you can imagine, before the radii are big enough to eliminate the undercut, they over lap at the base. I know there is a fillet feature that enables a set back and have no idea whether this is what i need, nor how to use it?

Thanks everyone,

Luke