。
I got pickpocketed on the bus.
Literal
I [topic-は] bus [possessive-の] inside [at-で] pickpocket [by-に] was-done-in.
スリにやられた — スリ ('pickpocket') + に (passive agent) + やられた (passive of やる, 'to be done in by'). やられる in this colloquial sense means 'to be victimized by.' A vivid, natural expression for petty crime.