Tries to divide existing practice (9)
I believe the answer is:
rehearsal
'practice' is the definition.
(rehearsal is a kind of practice)
'tries to divide existing' is the wordplay.
'tries' becomes 'hears' ('hear' can be a synonym of 'try').
'to divide' indicates putting letters inside.
'existing' becomes 'real' ('real' can be similar in meaning to 'existing').
'hears' put within 'real' is 'REHEARSAL'.
(Other definitions for rehearsal that I've seen before include "Preparation for a show" , "Dramatic practice" , "Drill" , "Practice session in preparation for public performance" , "Practice in theatre etc" .)