[SalesForce] How to call one controller function from another controller function in Lightning

You can definitely call another function within the same object using the this keyword. I just tested the following code and it works. The example doesn't make sense but it demonstrates the functionality of the code:

Component:

<aura:component >
    <input type="button" onclick="{!c.buttonClicked}" value="Click Me" id="myButton" />
</aura:component>

Controller:

({
    buttonClicked : function(component, event, helper) {
        helper.changeMyLabel(component, event.target.id);
    }
})

Helper:

({
    changeMyLabel : function(component, buttonId) 
    {
        var newLabel = 'My Label Changed';
        this.changeButtonLabel(component, buttonId, newLabel);  
    },
    changeButtonLabel : function(component, buttonId, newLabel)
    {
        document.getElementById(buttonId).value = newLabel;
    }
})

The handler name in your child component should match the registered name in the parent component. i.e., the handler in child component should be like this:

  <aura:handler name="clrValues" event="c:ClearValues" action="{!c.clear}"/>

--UPDATE--

I think a better approach for parent-to-child communication is to use <aura:method> as listed in lightning docs and here

To do this, you should define a method in child component like this:

  <aura:method name="sampleMethod" action="{!c.clear}" access="PUBLIC"> 

You should then have an id when you refer it inside your parent component like this:

  <c:ChildComponent textValue="test" aura:id="cComp"/>

Then in the clear method of the parent component, you should invoke the child method like this:

 var childCmp = component.find("cComp")
 childCmp.sampleMethod();