Eagle layout: how do I create an elongated hole under IC for isolation?
Use the "cut out" layer to draw the shape you need. With the board shop I work with, at least, the minimum width of the cut-out is 100 mils. 

The easiest way to do this is to draw a 100 mil wide line where you want your cut-out. You can either select the "cut out" layer before drawing the line or move it to that layer after it's drawn. It'll look something like this:



A full tutorial (where this image is from) is available at OSH Park, who will also make the boards for you rather inexpensively.