clutter up
To overcrowd a space or area with something. A noun or pronoun can be used between "clutter" and "up." I have so many books at home that they're starting to clutter up the place. Every time my kids come home from college for the summer, they clutter up the whole house with their stuff.
clutter something up
to mess something up; to fill something or some place up with too many things. Heaps of newspapers cluttered the room up and made it a fire hazard. Who cluttered up this house?