Clannish people prefer their own exclusive groups and aren't interested in outsiders. Because his red nose made him different, the clannish reindeer refused to let Rudolph join their games.
When you're new in school, it can feel pretty clannish, with established cliques that don't seem very welcoming to someone who's unfamiliar. Many groups behave in clannish ways, including some families, ethnic groups, religions, and even the theater kids at a high school. It's too bad, because it means they mainly hang out with people just like themselves and aren't exposed to new people and ideas. This adjective comes from clan, originally a Scottish tribe or family.