I had to introduce a princess in the theatre (3)
I believe the answer is:
ida
'princess in the theatre' is the definition.
(Princess Ida is a Gilbert and Sullivan opera)
'i had to introduce a' is the wordplay.
'i had' becomes 'id' (I'd).
'to introduce' means to remove the last letter (I've seen 'introducing' mean this).
'a' becomes 'an'.
'an' with its final letter removed is 'a'.
'id'+'a'='IDA'
(Other definitions for ida that I've seen before include "noted princess" , "- Rubinstein, Russian dancer, actress" , "Trojan Mount; G&S Princess" , "Classic mount" , "Mountain in Crete" .)