I want to create a Redstone clock that can be enabled and disabled with a lever permanently placed down.
Here's a selection of toggleable clocks of various periods and duty cycles - make your pick.
tuneable period of 5+ redstone ticks, about 50% duty cycle, just change the number of repeaters.
2rt period, 50% duty cycle, signal toggles between 15 and 2, so you need 2 extra redstone to make it actually switch on/off.
trivial observer clock, 2rt period.
4rt classic repeater clock, with observer on slime block to start it up instead of your torch.
trivial hopper clock, 15gt (7.5rt) period.
Fader, a slow clock with tuneable, long period; tuneable duty cycle (power level drops gradually on output; reduce it to cut-off threshold to tune the on:off ratio)
Etho clock. Period of up to 255 seconds depending on amount of items in the hoppers; 2rt off, remainder on duty cycle.