What does the cryptic crossword clue "Geeky type teaches at Hogwarts? (12)" mean?

The wordplay uses a charade (building blocks). TRAINS (synonym="Harry") + POTTER (synonym="Harry") = TRAINSPOTTER The answer is TRAINSPOTTER.

Geeky type teaches at Hogwarts? (12)

Guardian Cryptic #29467 | 14 Across | 2024-08-21 flower