Calling someone clever is the most important compliment you can give to anyone who's really smart. Smart is just smart. Clever means you thought up something important.  Way back in the 1970s I was in grad school and had a full scholarship doing research for my PhD advisor.  It was a great time and I had the chance to hang out in the lab with a really smart set of folks. Like the real world academic careers are not always fair and the smartest guy never got into a tenure track job; he ended up working in the lab ...