Spiderweb
noun
クモ is a spider. Add a nest to it, and you have a spider nest. What do spiders call their nests? They call it a spiderweb (that is, if they can talk).
くものす
クモのす
巣 is a standalone word here, so it uses the same reading that you learned when you learned the vocabulary.
屋根裏部屋で、クモの巣が掛かった祖父の未完成の絵を見つけたんです。
I found my grandpa’s unfinished painting covered in cobwebs in the attic.