Hi LLL
In short, I'd say "no". However, like with everything in this game there is a "it depends".
1. It depends on your definition of clean.
2. It depends on what real butter is.
For me, real butter is the gorgeous tasty, salty stuff like they used to make in the 1920's. The problem with real butter in this respect is that it is saturated fat. Saturated fat is animal fat, and the same stuff you find on the skins of chicken legs; the white bits of trim on bacon; the cream in a full-fat pint of milk; and the crackling on a pork joint. Now, in reality, the average Joe probably eats far too much of the stuff. However, for diet conscious bods, like yourself, you're probably keeping so far away from saturates in your diet, that a knob or two on a slice of bread will do no harm whatsoever.
Actually, all though it amounts to very little, we do need a teeny-incy-wincy bit of saturated fat for health. So, as long as you're not consuming copious amounts of saturated fat per week, I can see no harm in a teaspoon every now and then.
Craig