Concert one performs vocally is likely to turn out well (9)
I believe the answer is:
promising
'likely to turn out well' is the definition.
'concert one performs vocally' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'one' becomes 'i' (Roman numeral).
'performs vocally' becomes 'sing' (I am not sure about this - if you are sure you should believe this answer much more).
'prom'+'i'+'sing'='PROMISING'
'is' is the link.
(Other definitions for promising that I've seen before include "Encouraging" , "Showing great potential" , "Giving one's word" , "Favourable -- hopeful" .)