Somehow I undo it after a trial performance (8)
I believe the answer is:
audition
'trial performance' is the definition.
(I've seen this before)
'somehow i undo it after a' is the wordplay.
'somehow' is an anagram indicator.
'after' says to put letters next to each other.
'i'+'undo'+'it'='iundoit'
'iundoit' with letters rearranged gives 'udition'.
'udition' after 'a' is 'AUDITION'.
(Other definitions for audition that I've seen before include "Test for a role" , "Trial performance for an artiste" , "Hearing for actor competing for part" , "Hearing for a job-seeking actor" , "Test of a performer's suitability" .)