Upstart is temporarily hiding performer (7)
I believe the answer is:
artiste
'performer' is the definition.
(an artiste is a performer or entertainer)
'upstart is temporarily hiding' is the wordplay.
'hiding' indicates a hidden word.
'ARTISTE' is hidden in the letters of 'upstart is temporarily'.
(Other definitions for artiste that I've seen before include "professional dancer?" , "Performers" , "Tears it (anag.)" , "Trouper" , "Attires (anag.)" .)