Old person realities sent off the rails (9)
I believe the answer is:
israelite
'old person' is the definition.
(the Israelites are an ethnic group in the Bible)
'realities sent off the rails' is the wordplay.
'sent off the rails' indicates anagramming the letters (to go off the rails is to go crazy).
'realities' is an anagram of 'ISRAELITE'.
(Other definitions for israelite that I've seen before include "one of the chosen?" , "such as David?" , "Jew" , "of ancient tribes" , "For example, Nathanael" .)