Disappointing score, one inspired by new libretto initially (3)
I believe the answer is:
nil
'disappointing score' is the definition.
'nil' can be an answer for 'score' (I've seen this before). I'm not sure about the 'disappointing' bit.
'one inspired by new libretto initially' is the wordplay.
'one' becomes 'i' (Roman numeral).
'inspired by' is an insertion indicator (some letters breathed in or 'inspired' by others).
'initially' indicates taking the first letters.
The initial letters of 'new libretto' is 'nl'.
'i' inserted into 'nl' is 'NIL'.
Can you help me to learn more?
(Other definitions for nil that I've seen before include "Latin zero" , "Nothing found in senility" , "You get nothing" , "Nothing, zilch" , "duck" .)