Trio belt out part of opera (8)
I believe the answer is:
libretto
'opera' is the definition.
(thesaurus)
'trio belt out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'trio'+'belt'='triobelt'
'triobelt' anagrammed gives 'LIBRETTO'.
'part of' is the link.
(Other definitions for libretto that I've seen before include "Book of words of a musical" , "Opera's words" , "textbook" , "Book of words of an opera" , "Text set to music in opera" .)