I have created a poster using several objects throughout several layers. I would like to export the final document with a small, white padding. Can I use Inkscape's clipping functionality to create a rectangle that is slightly smaller than the page and then make everything outside of that rectangle transparent, such that a margin appears?
Yes, that is possible, but you will lose the layer structure if you use clipping, because the clip will force all objects into the same group in the same layer.
If you want to keep the structure, you can instead add a white frame on top of your drawing. For this, I would use snapping to snap a white rectangle to the page corners while drawing it, then I would duplicate that rectangle, and in the width/height fields subtract 2*margin, so you get a smaller version of that. Center it on the page and subtract that from the larger white rectangle. Put the frame on its own layer, that you can hide and show as needed.
You don't need to use clipping for this. In Inkscape a page is fully transparent by default, unless you put an opaque object on the page. So, you can use this to your advantage.
- Draw a white rectangle smaller than the page, and move it to the bottom of the stack using the Lower Selection to Bottom icon in the control bar along the top, or alternatively use the keyboard shortcut End. This will ensure all other content is on top.
- When you export, under Export area choose the Page button, then hit Export. The result will be a white rectangle within a larger transparent page.