A fuse was a component of an explosive.
In 2267, Spock selected a sonic grenade with a delayed action fuse from a munitions dump on Organia to distract the Klingons. (TOS: "Errand of Mercy")
In 2268, to confuse Norman, Harry Mudd called for a fuse when assembling an imaginary explosive. (TOS: "I, Mudd")