What does the cryptic crossword clue "Incur irritation of old characters (5)" mean?

The definition is "characters". The wordplay uses an anagram. anagram of INCUR = RUNIC; definition: "of old characters" The answer is RUNIC.

Incur irritation of old characters (5)

Guardian Cryptic #29437 | 9 Across | 2024-07-17 flower