Left bitter, possibly over text (8)
I believe the answer is:
Here is my best explanation:
'left bitter possibly over text' is the definition.
'libretto' can be an answer for 'text' (thesaurus). I'm not sure about the 'left bitter possibly over' bit.
'left bitter possibly over text' is the wordplay.
'left' becomes 'l' (common abbreviation).
'possibly' indicates an anagram.
'over text' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'bitter' with letters rearranged gives 'ibrett'.
(Other definitions for libretto that I've seen before include "textbook", "Words set to music, especially opera", "Text written for an opera", "component of opera", "Script for a musical drama", "Book of words of a musical", "Opera's words", "Words of a musical", "Book of words of an opera" and "The words of an opera or musical play".)