Tries to stop genuine practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'tries to stop genuine' is the wordplay.
'tries' becomes 'hears' (as in trying or hearing in a trial).
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'genuine' becomes 'real' (similar in meaning).
'hears' put inside 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Play run-through" , "Dummy run" , "Practice - run-through" , "Dramatic practice" , "Drill" .)