Tries to stop serious practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'tries to stop serious' is the wordplay.
'tries' becomes 'hears' (as in trying or hearing in a trial).
'to stop' means one lot of letters goes inside another.
'serious' becomes 'real' (similar in meaning).
'hears' placed into 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Dry run" , "Practice in theatre etc" , "preparation in house?" , "show trial" , "Dramatic practice" .)