Title Rob altered for opera text (8)
I believe the answer is:
libretto
'opera text' is the definition.
(I've seen this before)
'title rob altered' is the wordplay.
'altered' indicates an anagram.
'title'+'rob'='titlerob'
'titlerob' is an anagram of 'LIBRETTO'.
'for' acts as a link.
(Other definitions for libretto that I've seen before include "Opera's words" , "textbook" , "Book of words of a musical" , "component of opera" , "Operatic text" .)