Great tsar's favourite operatic piece (5)
I believe the answer is:
peter
'piece' is the definition.
(peter is a kind of piece)
'great tsar's favourite operatic' is the wordplay.
I cannot really see how this works, but
'favourite' could be 'pet' (synonyms) and 'pet' is found in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for peter that I've seen before include "One of Jesus' disciples" , "Disciple" , "- Sellers, twentieth century English comedian" , "foundation for church" , "Blue -, ship's flag" .)