Raster images are always rectangular (number of samples x number of lines). So clipping will always create the smallest bounding box it can around the vector, but it is still going to be a rectangular shape. If you want to mask the pixels outside of the vector boundary, then you can export the vector to an ROI and use Basic Tools ->Subset data via ROIs. This option allows you to mask areas outside the ROI to a value of your choice such as 0.