Norma, for one (5)
I believe the answer is:
opera
'norma for one' is the definition.
(Norma is an opera by Vincenzo Bellini)
This is the entire clue.
(Other definitions for opera that I've seen before include "Carmen, perhaps" , "Macbeth for one" , "Eg, Il trovatore" , "Aida, for example" , "eg Otello" .)