A clump is a "bunch" or a "cluster." If you're a gardener, the last thing you want to see is a clump of weeds growing in the middle of your newly groomed lawn.
Essentially, a clump is a grouping. You might see a clump of sheep grazing in a field or you might throw a clump of clothes into the washing machine. Clump can also mean lump, like when you find a clump of grass stuck to your shoe. As a verb, clump means "to gather," as in "a flock of birds clumped together in a tree."