The term 'messy substance' can be described by two crossword solutions: gunge (5 letters) and GLOP (4 letters). Gunge is a British slang term for a thick, slimy, and often smelly substance. It is usually composed of mud, oil, or other liquids. GLOP is an American slang term for a thick, gooey, and often messy substance. It is usually composed of food, paint, or other materials. Both terms are used to describe a messy substance that is difficult to clean up.