Incur irritation of old characters (5)

I believe the answer is:

runic

I'm a little stuck... Click here to teach me more about this clue!

'of old characters' is the definition.
(I've seen this before)

'incur irritation' is the wordplay.
'incur' can be anagrammed to 'RUNIC'.
However, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for runic that I've seen before include "Inscribed with ancient symbols" , "Relating to old Germanic alphabet" , "In a Norse alphabet" , "character hard to interpret" , "Like old Scandinavian writing" .)

I've seen this clue in The Guardian.

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