This person's after mind free from bias (9)
I believe the answer is:
objective
'free from bias' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'this person's after mind' is the wordplay.
'this person's' becomes 'I've' (as in 'I've', referring to the crossword setter).
'after' is a charade indicator (letters next to each other).
'mind' becomes 'object' (to mind something is to object to it).
'ive' after 'object' is 'OBJECTIVE'.
Can you help me to learn more?
(Other definitions for objective that I've seen before include "Unbiased - aim" , "end" , "Not showing any bias" , "Having real existence or not personal" , "Goal, aim, target" .)