Using mixer grinder circuit breaker in a electronic circuit
Most switching regulators (I assume it is switching given the 5A rating) have built-in over current protection.  This can be foldback current regulation where the voltage is reduced until the current drops to 5A, or hiccup where the output is switched off when the over-current condition is detected and then possibly gradually turned back on (via the slow-start ramp) after a timeout.  If the over-current condition still exists, then the hiccup cycle is repeated.

The circuit breaker would almost certainly work, but you may not know what the trip current is for the breaker (5A is probably the hold current where it will not trip), and you don't know how quickly the circuit breaker will activate on an over current condition.  If the step-down converter is limited to 5A, the circuit breaker is unlikely to every trip (it might on a short circuit as the energy in the output capacitor of the step-down converter is dumped but probably not).