Fifth wheel

SPARETIRE

It’s kept in the trunk for emergencies but isn’t a tool or a snack.

"Fifth wheel" is a playful way to describe something extra or unnecessary, like a spare tire in a car—it’s there as a backup but not actively used. The term also hints at the literal fifth wheel on some vehicles, reinforcing the idea of a spare. "Spare tire" fits perfectly as the answer, combining the wordplay of "fifth wheel" with a common automotive term.