I have a box full of clementines. Can I make orange juice in a blender?
Sure, you can juice with a blender, as long as (a) the blender is of reasonably good quality and (b) you're not expecting the same kind of yield or quality you'd get with a juicer (electric or manual). You'll also need a very fine strainer, or cheesecloth if you're like me and hate any amount of pulp.
You'll have to peel them first, and try to remove the pips as well (clementines shouldn't have very many, and might even be totally seedless). The pith is less of an issue - some people remove it, some people don't, it's a matter of personal preference. Once the oranges have been "cleaned", start blending them on the lowest speed and gradually bring it up to high. Oranges should have enough juice to blend on their own, but regardless of what fruit you use, be prepared to add water if the blender doesn't seem to be doing much.
Once you've got a fairly consistent purée, pass it through the strainer and mash the pulp against it with the back of a spoon or the bottom of a glass jar. There's your juice. You can throw some pulp (purée) back in if you like that texture.
Fruit purée from a blender doesn't really have the best flavour - it tends to turn out somewhat bitter - so you'll probably want to add a small amount of sugar and/or citric acid to compensate.
I would avoid that. Orange juice is normally simply squeezed. I would simply halve them and squeeze them. If you were to blend it and if you happened to get seeds in there then when you turn on the blender it will puree it which would add a funny taste to the juice.
Was your intent to get extra pulp or to just speed up the process?