One's past caring (9)
I believe the answer is:
historian
'past caring' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one's' is the wordplay.
I cannot quite understand how this works, but
'one's' could be 'is' (Roman numeral) and 'is' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for historian that I've seen before include "Writer about the past" , "Past master" , "student of the past" , "One sort of academic" , "So thin air suits recorder and analyst of past events" .)