Princess in opera is doing aria, first of all (3)
I believe the answer is:
ida
'princess in opera' is the definition.
(Princess Ida is a Gilbert and Sullivan opera)
'is doing aria first of all' is the wordplay.
'first of all' says to take the initial letters.
The initial letters of 'is doing aria' is 'IDA'.
(Other definitions for ida that I've seen before include "Greek mount - Gilbertian princess" , "Classical mountain" , "princess on stage" , "Classic mount" , "The girl for Dai" .)