Leaders admire bookish child showing what they've been learning in school (4)
I believe the answer is:
abcs
'they've been learning in school' is the definition.
I can't tell whether this definition defines the answer.
'leaders admire bookish child showing' is the wordplay.
'leaders' suggests taking the first letters.
The first letters of 'admire bookish child showing' is 'ABCS'.
'what' is the link.
Can you help me to learn more?
(Other definitions for abcs that I've seen before include "guides ..." , "12 [THINGS EASY]" , "Know these to be well versed - alphabetically?" , "Know these letters to be well informed" .)