Is there a way to hide those parts of a textbox that are outside the page margins? In the screenshot below, I want the line to stop at the margins. Or is there another (better?) way to achieve the same aim?
Select the text frame with the Selection Tool and Copy it.
Draw a rectangular picture frame
Edit > Paste Into
Set the picture frame to the margin width.
Or the way many may do it... draw white rectangles to cover things. A method I try and avoid.