Great hopes Berg cultivated about a stage work (3,7,5)
I believe the answer is:
the beggars opera
Here is my best explanation:
'a stage work' is the definition.
(opera by Johann Christoph Pepusch and John Gay)
'great hopes berg cultivated about' is the wordplay.
'cultivated' is an anagram indicator.
'about' becomes 'a' (abbreviation in Chambers).
'greathopesberg' anagrammed gives 'thebeggarsoper'.
'thebeggarsoper'+'a'='THE BEGGARS OPERA'