I'm trying to extract one single contour line at given elevation from a DEM. Taking into account that I want only one contour and not all contours with the given elevation, is there a way to do it in QGIS? What I really want it is a tool similar to the contour drawing in ArcView. Just one click on DEM or GRID into the main view e the contour is drawn. I'd like to reproduce something like that.
I think your best bet is to use
r.contour.level from GRASS in the Processing toolbox. This allows you to provide a list of specific contours (a list of one in your case). In this way you should be able to specify the individual contour you are interested in. The
Raster->Extraction->Contour tool requires you to specify just a contour interval which may not give you the control you need and will result in a lot of unwanted contours making it a little harder to pick out the one you want.
You said "I want only one contour and not all contours with the given elevation" which I take to mean that you will not want all contours the above process creates even though they will all be at your given elevation. So you have some options. The simplest is to select the individual contour you want and then save your contour vector file using
Save AS from the layer properties and tick the box "Save only selected features".