You can either assign the SCREEN-VALUE attribute of the widget to an empty string or the variable an empty string and display it. The first option does not clear the variable because for that you would need to assign it.
Option one ( only handles the screen value of the widget ):
DO WITH FRAME yourFrameName:
ASSIGN yourFillIn:SCREEN-VALUE = '"".
END.
Option two ( will clear the variable as well as the screen value ):
DO WITH FRAME yourFrameName:
ASSIGN yourFillIn = "".
DISPLAY yourFillIn.
END.
From the question you ask I'll take it that you have no understanding of screen buffer and record/variable buffer - you should make yourself familiar with them because otherwise you might run into troubles. Each widget has a screen and a variable buffer associated with it. You can set each of them individually and there are statements which set the one from the other like ASSIGN or DISPLAY. This is basic 4GL knowledge.
Heavy Regards, RealHeavyDude.