*Refreshes one's knowledge of

BONESUPON

Think of a phrase used when reviewing something you’ve already learned, often before a test or challenge—it’s two words mashed together.

The clue plays on the phrase "bone up on," which means to review or refresh your knowledge about something. "Bone" is split into "bones" here for a slight twist, and "up on" is tacked at the end to form "bones up on." It’s a light bit of wordplay where the answer sounds like a casual way to say you’re brushing up on a topic. The spelling might look odd at first, but it’s just the phrase written as one word for the crossword.