There is a straightforward reverse tool in FCPX that enables you to play a clip backwards. Flipping the clip 180 degrees is something different - if a character is looking left, flipping it means they're now looking right.
This is a great option for music video in particular. It can be used to effectively create additional shot variety. It can also be used for comic (or simply wow factor) effect when using more than one layer of a character or action.
I'll link to a student post when it becomes available, as this tool has been used to great effect for one of the videos from the 2015 Music Video Day (Christina Aguilera's Candyman), the viral-style green-screened video we previously would have shot on Creative Arts Evening.
Here's where I found the really simple (but ingenious!) instructions on how to do so - as the site seems to be partially blocked by the web filter, I'll include a screenshot below, but it is worth checking out the page, and maybe looking for other tips on this site.
Here's their short video on this: