Clue: What Snape does, being on Platform 9 3/4? (12)
Answer: TRAINSPOTTER
Explanation: Definition: being on Platform 9 3/4
Severus Snape is a teacher in the *Harry Potter* books, so he TRAINS POTTER, and a trainspotter might be a "being" on a platform; also in the books Harry takes the train to Hogwarts from platform 9 3/4 at Kings Cross station