Nurse treated old characters (5)
I believe the answer is:
runes
'old characters' is the definition.
(I've seen this before)
'nurse treated' is the wordplay.
'treated' indicates an anagram.
'nurse' anagrammed gives 'RUNES'.
(Other definitions for runes that I've seen before include "Old letters" , "Small stones bearing symbols with magic significance" , "Mysterious letters" , "old characters" , "marks on old tablets" .)