Cast finishes off brief rehearsal you began watching (5)
I believe the answer is:
flung
'cast' is the definition.
(cast can mean to throw or fling)
'finishes off brief rehearsal you began watching' is the wordplay.
'finishes off' suggests the final letters (take the finishing letters off of the words).
The final letters of 'brief rehearsal you began watching' are 'FLUNG'.
(Other definitions for flung that I've seen before include "Threw" , "Cast aside" , "Hurled" , "Thrown with force" .)