What children learn early: they can get through a door without opening it (7)
I believe the answer is:
letters
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'what children learn early they can get through a door without opening' is the wordplay.
I cannot really understand how this works, but
'get' could be 'let' (synonyms) and 'let' is present 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 letters that I've seen before include "eg E G" , "providing digs?" , "...as, for example" , "Characters" , "Missives" .)