Actress Shearer (5)
I believe the answer is:
norma
'actress shearer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for norma that I've seen before include "The 'Rule' constellation" , "Shearer or Major" , "Operatic heroine" , "Bellini work" , "John Major's wife - Bellini opera" .)