Made song about learner's jargon (5)
I believe the answer is:
slang
'jargon' is the definition.
(I know that jargon can be written as slang)
'made song about learner's' is the wordplay.
'made song' becomes 'sang' (sang a song).
'about' means one lot of letters goes inside another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'sang' enclosing 'l' is 'SLANG'.
(Other definitions for slang that I've seen before include "... to scold" , "Vernacular" , "Informal language - jargon" , "Racy speech" , "Insult" .)