Letter-writing skill of witches and wizards? (8)
I believe the answer is:
spelling
This clue is a double definition.
'letter-writing' is the first definition.
(spelling is writing out letters)
'skill of witches and wizards?' is the second definition.
(witches and wizards cast spells)
(Other definitions for spelling that I've seen before include "Taking turn" , "Competition to list the letters of a word" , "English teacher's subject, perhaps" , "Giving letters (of word)" , "component of education" .)