Opera in Carmen or Manon (5)
I believe the answer is:
norma
'opera' is the definition.
(I've seen this before)
'in carmen or manon' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NORMA' can be found hidden inside 'carmen or manon'.
(Other definitions for norma that I've seen before include "She incited Gauls v. Romans (Bellini)" , "Marilyn was properly one" , "Roman (anag) -- opera" , "Drawing instrument" , "Bellini drama" .)