A performer in taking part is terrible (7)
I believe the answer is:
artiste
'a performer' is the definition.
(artiste is a kind of performer)
'in taking part is terrible' is the wordplay.
'in' indicates a hidden word.
'ARTISTE' is hidden within 'taking part is terrible'.
(Other definitions for artiste that I've seen before include "Attires like entertainer" , "Player" , "Performers" , "Person playing" , "A sitter for creative person" .)