There are almost always multiple ways to accomplish things in Illustrator; I will present one way to do this.
Here we have a closed path (Pen Tool shape) in red above grouped paths:
Select the Shape Builder Tool and drag it through the interior of the upper path until the whole area is shaded
then opt/alt-click any remaining line segments to remove them.
You will be left with the upper shape, empty, and the outside remainder of the grouped paths.
If you wish, you can select and delete the upper path:
Alternatively, if you know you want to get rid of the upper path you can opt/alt-drag with the Shape Builder through the interior of the upper shape to delete all the parts you've dragged through, including the upper shape perimeter, as soon as you release the mouse button.