It depends on whom you are talking to. Both opera f. sg. and opus in the musical sense are obviously transparent to anyone born after 1700, and I'd go with it in a modern text.
If you are explaining opera to a Roman, though, consider how much music and singing the classical theater involved already. To them, the opera would certainly be a kind of drama, so your best bet would be to emphasize the properties that distinguish it from the “default,” central definition of the concept. I can think of two major ones: music as a significant feature, and the singing. Both were already prominent in the classical theater (I am reminded of the Tacitus' account of Nero playing lyre and singing; it made me chuckle), so it perhaps should not take a massive verbiage to amplify one or both.
For the emphasis on music, Joonas' melicum sounds good to me. I cannot, however, think of a single classical word that would oppose singing to reciting, e. g. carmen is both a song and a poem, and cano is both sing and recite; it seems to me that these concepts were not separate in the Roman mind. Cic. and later Plin. used cum symphonia canere to describe singing.
The most concise way to convey both emphases that I can think of is the adj./p.p. modulatum, which, in general, implies adding rhythm to an artistic performance, thus drama modulatum for “opera”. (Note that modulatum has a side effect that it also might imply dancing, which was also present in the dramatic κατάβασις, but this would not probably disqualify it.) All things considered, I imagine that a concise introduction to the concept of modern opera along the lines of drama musicā symphoniaque modulatum quid nostri operam vocat would paint a close enough picture to a Roman of the classical times.