A clump is a compacted mass. Ron and Harry (alongside Hermione) are supposed to be a part of a single functioning unit, which Hermione is comparing to a clump. Because Harry and Ron don't get along, the clump is unhappy. That's what Hermione means when she complains about them not clumping happily.
The reason Hermione is considered the Only Sane Man ("the reasonable Hermione") is that she's the only one who never engages in cannibalism (Harry never succeeds in eating anyone onscreen, but he seems to be considering eating Hermione) and is the only one of the three not to be distracted by infighting the way Ron and Harry are.
While Hermione is unfazed by Ron eating her family, after Harry dips her in hot sauce, hungrier than he's ever been, presumably to eat her, she's still there with Harry and Ron singing about how there's no way to get rid of them. This indicates that — unless she managed to escape being eaten — it's possible to survive being eaten in this universe and that's why Hermione is able to respond to the cannibalization of her parents with Angst? What Angst?.
