Wiktionary
vb. to become clumpy, to form into a clump
Usage examples of "clump up".
Then you'd ship me back to the orphanage, and I'd be so emotional about it, I'd clump up to the roof and throw myself off, a most tragic suicide, which none of us really would want to see happen.
Uncle Ron would clump up the stairs heavily and there would be a scurrying, with suppressed giggles and muffled screams, until everyone found a bed again, so that by the time he turned on the hall light that illuminated the attic dimly, all the children would seem to be sleeping.
Sometimes even the most independent needed to clump up, hip and shoulder touching, protecting their soft flanks from the wolves.
The young butler could feel his hair trying to clump up and push out in spikes as those empty eyes-eyes like dead lamps lit only by the moon-passed over him.