・。
Maria Callas was a famous opera singer.
Literal
Maria Callas [topic-は] famous opera singer [copula-です-past-でした].
A simple nominal-predicate sentence with past-tense copula でした. オペラ歌手 is 'opera singer' — a compound of the loanword オペラ + native 歌手 ('singer'). This mix of foreign loan + native word is typical for specialist vocabulary.