僕は彼女の歌のピアノ伴奏をした。

。

I played the piano accompaniment for her singing.

Literal

I [topic-は] her [attributive-の] singing [attributive-の] piano accompaniment [object-を] did.

伴奏 ('accompaniment,' specifically musical). ピアノ伴奏 ('piano accompaniment'). The chain of の links 彼女 → 歌 → ピアノ伴奏. ピアノ is an Italian loanword (from pianoforte), though it entered Japanese via European contact.