They use librettos in a prose translation (6)
I believe the answer is:
operas
'they use librettos' is the definition.
The definition suggests an adverb but the answer is not.
'a prose translation' is the wordplay.
'translation' is an anagram indicator.
'a'+'prose'='aprose'
'aprose' is an anagram of 'OPERAS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for operas that I've seen before include "The Magic Flute, Don Giovanni etc" , "Nabucco and Tosca" , "O spare these dramas" , "These shows are a poser, oddly" , "Dramas set to music" .)