Most of standard opera (5)
I believe the answer is:
norma
'opera' is the definition.
(I've seen this in another clue)
'most of standard' is the wordplay.
'most of' means to remove the last letter.
'standard' becomes 'normal' (similar in meaning).
'normal' with its last letter taken away 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" .)