What Gilbert wrote, demolishing litre bottle? Not half! (8)
I believe the answer is:
libretto
'half' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for libretto that I've seen before include "Operatic book" , "Book of words of a musical" , "Text written for an opera" , "text for musicians" , "The words of an opera or musical play" .)