Rehearsal is terminable at intervals (5)
I believe the answer is:
trial
'rehearsal' is the definition.
'terminable at intervals' is the wordplay.
'at intervals' indicates alternate letters (letters are taken at regular intervals).
The alternate letters of 'terminable' are 'TRIAL'.
'is' acts as a link.
(Other definitions for trial that I've seen before include "Hearing - test" , "Experiment - adversity" , "Court examination; test" , "Testing, experiment" , "Court appearance on charge - it's an ordeal" .)