Instructs wizard in anorak (12)
I believe the answer is:
trainspotter
'anorak' is the definition.
(I've seen this before)
'instructs wizard' is the wordplay.
'instructs' becomes 'trains' (training is a kind of instructing).
'wizard' becomes 'potter' (Harry Potter).
'trains'+'potter'='TRAINSPOTTER'
'in' acts as a link.
(Other definitions for trainspotter that I've seen before include "Nerd" , "anorak" , "Boring person" , "Gricer" , "One interested in engines" .)