It's a 2 step process. Step 1: Create a line item "Zero out" in your Target module and load this value into your target line item. This will clear out the existing values. Zero Out --> Demand, in the same module
Summary of the new line item is none and formula is 0.
I agree with @LipChean_Soh , You should use the standard clear option, you could simply clear 'All items' in the Line items tab. That is like beginning with a clean slate every time you run the action.
What if I want to clear only one line item and don't want to disturb the other line items(Manual inputs). So it is best practice to use 'All mapped items" so that you will not loose any other data.
@MadhabikaM If you want to create an action, go with the approach of creating line item and make it as No data as this will save your model size(This will clear out for any formatted line item ex. Number, Text, Boolean, list etc.) and import into your desired line item.