1. "Spelt" is a verb here. (The other version of this in the US is "spelled.") It refers to the the arrangement of letters used to form a word.
2. This phrase ("spelt priest") is referring to the order of letters used to form the word or last name "priest." In the examples below, the word refers to someone's surname/ last name. In other cases, a "priest" is a religious occupation/ authority figure in a church, etc.
“In the modern idiom the name is variously spelt: Priest, Preist, Prest, Prestt, Press…”
“The name is also spelt PRIEST, PREIST and PRESTE.”