。
She trained as a violinist under a famous musician.
Literal
She [topic-は] famous musician [possessive-の] under [at-で] violin player [as-としての] training [object-を] received.
~の下で ('under [someone]') marks a teacher-student relationship. としての marks the capacity ('as a violinist'). 訓練を受ける ('to receive training') is a formal expression. バイオリン奏者 (そうしゃ, 'player/performer') uses the formal suffix ~者.