Opera villainess, often (4)
I believe the answer is:
alto
'opera villainess often' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alto that I've seen before include "Highest male voice: lowest female voice" , "Singing range" , "Lowest female singing voice" , "Singer's hidden inside chemical toilet" , "Key old > male who scores high" .)