how do I crop an artboard in Illustrator?

I would first zoom (max 64000%) in to the effected area and see if it still appears to be there, there may be a preview issue, if so toggle Ctrl+E (GPU) and see if that has an effect

Secondly check your x and y's and do a test with a solid object and max it out over the artwork and see if you are still having the same. Also check to see if align to pixel grid is on which may shift it out of place great article here Re: What is happening with these pathfinder bugs?.

Lastly, if it is indeed an image you are importing then just make it larger than the background and then mask it to size within Illy.

I agree with cwalker, I would look for stray objects first, but a sliver of white doesn't make me think its a stray object or points.  cropping the document would not really be a fix unless you didn't care about the size of the doc. I imagine you are pulling an image in and it doesn't fit the doc and you want the artboard to match the image maybe? in any case, edit your artboard.

CMD+Option+p(MAC)   CTRL+ALT+P(PC)

Then click "Edit artboards"

you can re-size individual artboards or choose exact size or other optinos in the menu bar.

cwalker,

As I (mis)understand it, you may try Ctrl/Cmd+A and see whether there is something invisible out there (such as stray points), and get rid of it (you may see what it is in the Layers palette (or elsewhere) if you select it separately).

use the artboard tool