Trio belt out opera text (8)
I believe the answer is:
libretto
'opera text' is the definition.
(I've seen this before)
'trio belt out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'trio'+'belt'='triobelt'
'triobelt' is an anagram of 'LIBRETTO'.
(Other definitions for libretto that I've seen before include "Text set to music in opera" , "text for musicians" , "The words of an opera or musical play" , "Words set to music, especially opera" , "Text written for an opera" .)