Professional baritone upset one being tried out (11)
I believe the answer is:
probationer
'one being tried out' is the definition.
'professional baritone upset' is the wordplay.
'professional' becomes 'pro' ('pro' can be a synonym of 'professional').
'upset' indicates anagramming the letters.
'baritone' is an anagram of 'bationer'.
'pro'+'bationer'='PROBATIONER'
(Other definitions for probationer that I've seen before include "Offender on conditional release" , "New employee" , "one under legal supervision?" , "Person given Test" , "Novice" .)