I've seen someone on YouTube create a LEGO Clock with all the wheels and a pendulum, but I was wondering if there is any project or building instructions available to build such a clock with an efficient movement. LEGO Technic seems ideal to build such complex mechanism.
Building a clock is easy, building an accurate clock is hard, even without lego.
To have accuracy you must have something happening at a fixed frequency. A pendulum is the easiest way to provide a regular timing.
Next you need a power source for the clock. Weights falling down is classic, and easy to do with lego : a pulley and a string.
The force generated by a spring is not constant, it decrease with time as the spring wound down. You need a constant force to have accuracy. Real clocks have mechanisms to compensate this variations by I am not sure it is possible to replicate with lego.
An electric motor is better, but the charge of the batteries determine the power generated by the motor.
Then you will loose power with friction so you must limit the number of moving part or limit friction with lubricant for example.
I have built a clock myself, it works but stops after 15 to 20 minutes for too much friction. You can search the web for other lego clocks, but the best I have seen works for about 6 hours.
A lego clock is a fun project but you won't use it to tell time.
According to this video of tuning a LEGO clock by KEvronista, he was able to get the accuracy within a few seconds per day. The key is to use a small amplitude of oscillation (angle) for the pendulum swing, so that the pendulum approximates a harmonic oscillator. The LEGO clock has an amplitude of about 2.3° degrees. Also important is making the drive chain and pendulum swing as independent as possible by reducing recoil, which the Galileo's escapement shown has as a major disadvantage. The Wikipedia article on the anchor escapement gives a good explanation and in theory a well-constructed deadbeat escapement made in LEGO could be even more accurate.