One's early teaching is keeping one down-to-earth (9)
I believe the answer is:
grounding
'one's early teaching is' is the definition.
I can't tell whether this definition defines the answer.
'one down-to-earth' is the wordplay.
'one down' becomes 'ing' (I am not sure about this - if you are sure you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'earth' becomes 'ground' (synonyms).
'ing' after 'ground' is 'GROUNDING'.
'keeping' is the link.
Can you help me to learn more?
(Other definitions for grounding that I've seen before include "Punishment by parent" , "Rudimentary knowledge" , "hampering progress of high flier?" , "Foundation of basic knowledge" , "keeping home" .)