Performer taking part is terrific? Not entirely (7)
I believe the answer is:
artiste
'performer' is the definition.
(artiste is a kind of performer)
'part is terrific? not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'ARTISTE' is hidden within 'part is terrific'.
'taking' is the link.
(Other definitions for artiste that I've seen before include "Tears it (anag.)" , "Professional performer" , "Theatre performer" , "A sitter for creative person" , "Performers" .)