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' with letters rearranged gives 'OPERAS'.
'in' is the 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" .)