the reset button would need to be in its own form. you would want to add a hidden element to the form.
Then add code that is triggered on the hidden element to clear whatever variable is used for the dynamic list.
i cant really give specifics without knowing ow the list is set up. select the list in design view, then click the dynamic button in the property inspector, what variable is used for the select item equal to option? this is the variable that will need to be unset when the reset button is pressed.