Bellini opera is not quite standard (5)
I believe the answer is:
norma
'bellini opera' is the definition.
(I've seen this before)
'not quite standard' is the wordplay.
'not quite' means to remove the last letter.
'standard' becomes 'normal' ('normal' is associated in meaning with 'standard').
'normal' with its last letter taken away is 'NORMA'.
'is' is the link.
(Other definitions for norma that I've seen before include "Bellini's heroine" , "name for Monroe" , "Operatic work" , "small group of stars" , "She incited Gauls v. Romans (Bellini)" .)