Anorak wearer possibly teaches at Hogwarts (12)
I believe the answer is:
trainspotter
'anorak wearer possibly' is the definition.
(trainspotters stereotypically wear anoraks)
'teaches at hogwarts' is the wordplay.
'teaches' becomes 'trains' (training is a kind of teaching).
'at' means one lot of letters go next to another.
'hogwarts' becomes 'potter' (I can't justify this - if you can you should believe this answer much more).
'trains'+'potter'='TRAINSPOTTER'
(Other definitions for trainspotter that I've seen before include "Gricer" , "Nerd" , "anorak" , "One interested in engines" , "Boring person" .)