I think it depends on the person. I have met some people with drug problems who can safely drink.
For me though, I think clean and sober have to be both. Although alcohol is what I finally came into the program for, it is really one in a series of additive behaviours and substances. Before I was an alcoholic, I was a pothead and only switched to alcohol when I moved to a country where I could not easily get pot. So yeah, for me, no unprescribed drugs or alcohol is what clean and sober means.