I use www.lunapic.com
Under 'Edit' you can choose 'Eraser Tool' to hand-scrub out the areas around it, or you can choose 'Transparent Background' if it's one solid color background that you want to just click on to make transparent. Then you save the image, and you have a clipart.
You can also use the same site to crop images first, to reduce the size so there's less to erase.
Hope that helps! :-)