1. To form into clustered masses or lumps. The mixture will begin to clump up if you don't stir continuously. The grass will clump up inside the lawnmower if you cut it while it's wet.
2. To cause something to form into a clump or clumps. In this usage, a noun or pronoun can be used between "clump" and "up." You've got to make sure no dirt gets into the seal, or it could clump up the lubricant inside the machinery. Don't just clump up your suit jacket like that. It'll get all wrinkled!
See also: up
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.