Most of standard opera (5)
I believe the answer is:
norma
'opera' is the definition.
(Vincenzo Bellini opera)
'most of standard' is the wordplay.
'most of' means to remove the last letter.
'standard' becomes 'normal' (associated in meaning).
'normal' with its last letter taken off is 'NORMA'.
(Other definitions for norma that I've seen before include "Operatic heroine" , "Roman (anag) -- opera" , "Bellini opera - girl's name" , "Drawing instrument" , "Game; shared supply" .)