char empty[] = " ";
sprintf (RemoteXY.edit_1, "%s ", empty);

No sadly not possible

There may be a workaround but I have never tried if it works: set the text color of the Edit field element so that it is invisible (same color as background), then on top of the Edit field, add a Text String element. Then in your code, constantly get the text in the Edit field and copy it into the Text String. Then you can clear the Text String whenever you want so that it appears that the Edit field is also cleared (because its text is invisible). Of course the problem is, it won't clear the text that is displayed inside the popup window.

Edit: This feature was added recently, see here http://forum.remotexy.com/viewtopic.php?id=1034

Is there a way to clear an Edit Field just after text entered into it ?