I suspect you are trying to override another number formatted line item with a input line item but problem you are facing here is you wont be able to override a value with zero.
Here is a workaround for you, Keep the line item as text formatted and use another line item with formula Value(Text formatted line item) this will convert your text to number to use it further and default value will be blank also when you try to put zero it will take it.
Re: Is it possible to override number formatted line item default value to BLANK
Anaplan always considers "ZERO" as the default value for any number formatted line item.
Considering one of our use case, I am anticipating your use case would also be similar.
We were build an override feature that allows users to explicitly override any demand with "0". In to address this feature we had 2 options:
Option 1 : Create a boolean with same dimensions of the line item. Use this boolean as a Dynamic Cell Access on the numbered line item. User would then be able to select the boolean as TRUE and only then the numbered line item would show a value of "0" or he could edit the value. If the boolean is FALSE, the numbered line item would show BLANK. (Most Recommended)
Option 2: Ask users to Enter an explicit value of "-1" in order to consider a ZERO override. And use "-1" in downstream calculations. (Please do not forget to add notes on the line item, in case you think this is better approach).