I have an unused 3 wire 220V 55 amp circuit from the meter from my old A/C unit running right where I need a 20 amp 3 wire 220v, is there a way to reduce the circuit, put in a remote 20 amp breaker safely without changing the main breaker?
Your current circuit, going to a large air conditioning unit, is almost assuredly 2-wire hot-hot + ground.
The important part here is that you will not be able to power a load that requires hot-hot-neutral (and often ground). This is very important. You cannot re-task the current ground wire to be a neutral.
However, most of the 20A 240V loads I can think of do not require a neutral. If they plug into a NEMA 6 type receptacle, you are all set.
Change the breaker —— The best way to do this is change the circuit breaker in the main panel which feeds this circuit. If there is still a shutoff switch, fuse or second breaker that is 50A, that does not matter. The breaker in the main panel will sufficiently protect the circuit.
You can then attach it to a NEMA 6-20 receptacle. These come in both single or doubles. Read the documentation with your 240V device; if it requires a dedicated circuit use a single receptacle, otherwise feel free to use a double.
You may have trouble attaching the heavy 6 AWG wire to a 20A breaker or receptacle. If so, use a 12 AWG pigtail. You can attach the two wires with a large (red or tan) wire nut.
You could certainly add a 20A breaker in a box somewhere down the line, but if you don't use the line at all at present, why not just swap the breaker from 55A to 20A for the time being? Depending on the terminals you might need to have a pigtail to fit the 20A breaker and join to the current wire, but that's not a big deal. If you later develop some additional use, you can address wiring for that, at that point.