Callas transformed a famous opera venue (2,5)
I believe the answer is:
la scala
'opera venue' is the definition.
(Milanese opera house)
'callas transformed a' is the wordplay.
'transformed' is an anagram indicator.
'callas' anagrammed gives 'lascal'.
'lascal'+'a'='LA SCALA'
'famous' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for la scala that I've seen before include "Italian opera house" , "noted venue?" , "Milanese opera house" , "A Callas may find way to Milan's famous opera house" , "where Toscanini was musical director" .)