Learns in the course of proper dummy run (9)
I believe the answer is:
rehearsal
'dummy run' is the definition.
(I've seen this before)
'learns in the course of proper' is the wordplay.
'learns' becomes 'hears' ('hear' can be a synonym of 'learn').
'in the course of' is an insertion indicator.
'proper' becomes 'real' ('real' can be similar in meaning to 'proper').
'hears' inserted inside 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "show trial" , "Dummy run" , "Dramatic practice" , "Drill" , "Play run-through" .)