A performer in a difficult part is tense (7)
I believe the answer is:
artiste
'a performer' is the definition.
(an artiste is a performer or entertainer)
'in a difficult part is tense' is the wordplay.
'in' says the answer is hidden in the clue.
'ARTISTE' is hidden in the letters of 'difficult part is tense'.
(Other definitions for artiste that I've seen before include "Player" , "Attires for professional entertainer" , "Trouper" , "A sitter for a performer" , "'Public performer, dancer or singer (7)'" .)