To mistrust is to feel uneasy about someone or something. If you mistrust bus drivers, you may decide to walk instead.
Are you suspicious of clowns in general? Then you mistrust them. This verb has a very similar meaning to distrust — as nouns, both words mean "a lack of trust." A subtle difference between the two is that distrust tends to be based on a bad experience (if a clown once pinched you, you will probably always distrust that clown). Mistrust, on the other hand, is a more general sense of suspicion.