Clutter is always a messy business!
In my world and in my book, I call clutter muck. Very often clutter is not visible. This is also why as a professional organizing generalist, I organize space, time, data and objects. Muck is anything that makes you stuck or unable to move forward. It does not have to be messy or a pile of junk. It can be an overcrowded schedule or an old belief that no longer serves you.