Rambling Roman opera (5)
I believe the answer is:
norma
'opera' is the definition.
(I've seen this before)
'rambling roman' is the wordplay.
'rambling' is an anagram indicator.
'roman' is an anagram of 'NORMA'.
(Other definitions for norma that I've seen before include "small group of stars" , "Marilyn Monroe's real first name" , "Eponymous woman of opera" , "John Major's wife - Bellini opera" , "Drawing instrument" .)