What does the slogan refer to, there are many meanings of the word clear.

It's good, desirable and in fashion to be understandable. Some people think that somebody that is hard to understand is somehow to be admired, since they must know something we don't. The sentence here says that it is actually more desirable and admirable to be clearly understood. That is a standard the rest of us should try to achieve.

7-Up's slogan "It's Cool To Be Clear", which was accompanied by a mascot designed to be the epitome of cool (at least for 1990's) The commercial features its mascot as a cartoon of a typical teenager in t-shirt and shorts who is supposed to represent the fashionable sporty guy who is "cool looking and clear" about the taste of 7Up.