When she was five, Harumi lost her parents in a car crash. It was an uncanny collision; a truck crossed over the median into their lane. Harumi was in preschool, rehearsing for a school play. She had no recollection of when she heard the news. She was sure she’d been overwhelmed with sadness, but the memory had slipped free in one clean piece. (Source - Higashino: The Miracles of the Namiya General Store)
I figure that "had slipped free in one clean piece" can be paraphrased as "had completely disappeared", but I want to know the exact meaning of the words "free" and "clean" in this context, or this whole phrase is a collocation. Moreover, I know "in one piece" means "without being hurt or damaged", but that doesn't make much sense here, so I would appreciate some explanation.