No, it is not true that a microwave's clock uses more power in its lifetime than the power required to heat food. The microwave's clock is a simple electronic component that consumes very little power compared to the actual microwave heating process.
The power consumption of a microwave's clock is typically in the range of a few watts or less. On the other hand, the microwave's heating function consumes significantly more power, usually in the range of hundreds to over a thousand watts, depending on the microwave's wattage and the cooking settings.
When you use a microwave to heat food, the microwave's heating element draws power from the electrical outlet to produce microwave radiation, which in turn heats the food. This power consumption during the cooking process is substantially higher than what the clock uses over the entire lifetime of the microwave.
In summary, the power consumed by the microwave's clock is negligible compared to the power used for cooking food. The primary energy consumption in a microwave's lifetime comes from the cooking process itself, not from the clock.