Fell had a sudden insight. in some devious way, with his books and commentary, sobel had given miriam to understand that he loved her. the shoemaker felt a terrible anger at him for his deceit. why is this passage from “the first seven years” by bernard malamud an example of an epiphany?

An epiphany is defined as 'an illuminating discovery or realization'. 

In this passage, Fell realizes that Sobel has let Miriam know that he is in love with her. The reader knows that it is an epiphany because the knowledge hits Fell so quickly. The passage says he had a "sudden insight", so we know that this realization happened in that instant, like an epiphany does.