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