Teacher here starts from old-fashioned concept to form belief system (6,2,7)
I believe the answer is:
school of thought
'form belief system' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'teacher here starts from old-fashioned concept' is the wordplay.
I cannot really see how this works, but
'teacher' could be 'school' (school teacher is a kind of teacher) and 'school' is found within the answer.
'from' could be 'of' (of can mean from or belonging to) and 'of' is found within the answer.
'old' could be 'o' (abbreviation) and 'o' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for school of thought that I've seen before include "Philosophical Academy?" , "Approach" , "Particular way of thinking" .)