Number one in school for language (6)
I believe the answer is:
speech
'language' is the definition.
('speech' can be a synonym of 'language')
'number one in school' is the wordplay.
'number' becomes 'pee' (I can't justify this - if you can you should believe this answer much more).
'one in' is an insertion indicator.
'school' becomes 'sch'.
'pee' put into 'sch' is 'SPEECH'.
'for' is the link.
Can you help me to learn more?
(Other definitions for speech that I've seen before include "Oral form of communication" , "Speaking faculty" , "'Utterance, talk (6)'" , "Communication by word of mouth" , "Vocal communication" .)