She is the third soprano principally engaged in Evita (6)
I believe the answer is:
person
'she is the third' is the definition.
('she' is a third person pronoun)
'soprano principally engaged in evita' is the wordplay.
'principally' indicates taking the first letters.
'engaged in' is an insertion indicator.
'evita' becomes 'peron' (nickname of Eva Peron).
The initial letter of 'soprano' is 's'.
's' going inside 'peron' is 'PERSON'.
(Other definitions for person that I've seen before include "A man or woman" , "he's the third" , "Individual human being" , "Bod" , "character" .)