@Fikret_Huseynkhanov i can confirm that this is not possible. Retool simply hasn't exposed the method to clear the inputValue property.
I even tried placing the multiselect inside a form, and the resetting the form. Did not work.
In case this has to do with your previous query (adding custom val to multiselect), let me know the exact outcome you want to achieve, and i might be able to suggest an alternative method.