Not entirely routine opera (5)
I believe the answer is:
norma
'opera' is the definition.
(Vincenzo Bellini opera)
'not entirely routine' is the wordplay.
'not entirely' means to remove the last letter (most but not all of the word).
'routine' becomes 'normal' (I've seen this in another clue).
'normal' with its last letter taken away is 'NORMA'.
(Other definitions for norma that I've seen before include "Shearer or Major" , "small group of stars" , "Bellini work" , "She incited Gauls v. Romans (Bellini)" , "Operatic work" .)