Learns in the course of proper practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'learns in the course of proper' is the wordplay.
'learns' becomes 'hears' ('hear' can be a synonym of 'learn').
'in the course of' means one lot of letters goes inside another.
'proper' becomes 'real' ('real' can be similar in meaning to 'proper').
'hears' inserted within 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Dummy run" , "Practice session in preparation for public performance" , "show trial" , "Dramatic practice" , "Dry run" .)