Tosca and Lucia Di Lammermoor (6)
I believe the answer is:
operas
'tosca and lucia di lammermoor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for operas that I've seen before include "They're written by 1 across" , "Musical plays" , "'La Boheme and La Traviata, say (6)'" , "Dramatic works set to music" , "Musical performances" .)