Be sassy, with “off”

MOUTH …

Think of a slang term for talking back or being cheeky—it’s often paired with a body part that does the talking.

This clue plays on the phrase "mouth off," which means to talk back or be sassy. The word "off" is given right in the clue, so you just need to add "mouth" to complete the expression. It’s a straightforward fill-in-the-blank with a bit of slang—no hidden tricks, just a common idiom split up.