Left bitter, possibly over text (8)
'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'.
Can you help me to learn more?
(Other definitions for libretto that I've seen before include "textbook", "Book of words of a musical", "component of opera", "Script for a musical drama", "Text written for an opera".)