Of an ancient alphabet (5)
I believe the answer is:
runic
'of an ancient alphabet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for runic that I've seen before include "In a Norse alphabet" , "Like mysterious characters" , "Pertaining to an ancient Germanic alphabet" , "Like some old characters" , "Like old Scandinavian writing" .)