This is a double definition - the clue contains two separate definitions that both mean the same answer word.
Both 'Class' and 'protocol' can mean the same thing. Think of a word that means both a category or type of thing, and also proper procedure or etiquette.
Answer: FORM | This is a double definition: 'Class' = FORM (as in a class at school, or type/category) and 'protocol' = FORM (as in proper form or procedure)