Catches taken in serious practice session (9)
I believe the answer is:
rehearsal
'practice session' is the definition.
(rehearsal is a kind of practice session)
'catches taken in serious' is the wordplay.
'catches' becomes 'hears' (catch can mean to hear or pick up).
'taken in' is an insertion indicator.
'serious' becomes 'real' (similar in meaning).
'hears' going inside 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Dramatic practice" , "Dry run" , "Play run-through" , "Practice session in preparation for public performance" , "preparation in house?" .)