Catches held in proper practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'catches held in proper' is the wordplay.
'catches' becomes 'hears' (catch can mean to hear or pick up).
'held in' indicates putting letters inside.
'proper' becomes 'real' (similar in meaning).
'hears' placed within 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Drill" , "Play run-through" , "Practice session in preparation for public performance" , "Practice - run-through" , "Dummy run" .)