Tries during proper practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'tries during proper' is the wordplay.
'tries' becomes 'hears' (as in trying or hearing in a trial).
'during' is an insertion indicator.
'proper' becomes 'real' (similar in meaning).
'hears' put within 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Practice session in preparation for public performance" , "Dramatic practice" , "Dry run" , "Dummy run" , "show trial" .)