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Rap on the starter itself with a large screwdriver handle.
If that makes the starter work, replace it.
In general, starters are hard-wired to the battery.
When you turn the key to Crank, the ignition switch routes power through your clutch switch to the solenoid on the fender. That solenoid is likely the click you are hearing.
From the solenoid, power is sent to the starter to engage an electromagnet and mechanically close a large switch inside the starter. That allows the power from the battery to turn the electrical starter motor and mechanically makes the gear (sometimes called a bendix gear) mesh with the flywheel.
In yours, the electromagnetic solenoid inside the starter is not making that full throw.
Many times, you can remove the starter (after disconnecting the Battery!) and douche it out and grease that gear's shaft that it slides on. May or may not help. The other common failure mode is the brushes get too short to make electrical contact and the motor never spins.
In short, I would remove the starter and have it tested at the auto parts store. It is possible to get new brushes. Inside the starter is not very complex. Getting the internal parts can be a bother.
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