Spelling system that uses first half dozen letters (3)
I believe the answer is:
hex
This clue is a double definition.
'spelling' is the first definition.
(a hex is a curse or spell)
'system that uses first half dozen letters' is the second definition.
(hexadecimal counting uses A-F as well as 1-9)
(Other definitions for hex that I've seen before include "Witch" , "Bewitch" , "Wicked spell" , "A magic spell or curse" , "Evil spell" .)