Performer in part is terrible (7)
I believe the answer is:
artiste
'performer' is the definition.
(an artiste is a performer or entertainer)
'in part is terrible' is the wordplay.
'in' indicates a hidden word.
'ARTISTE' can be found hidden inside 'part is terrible'.
(Other definitions for artiste that I've seen before include "Attires (anag.)" , "Player" , "Attires like entertainer" , "A sitter for creative person" , "Trouper" .)