Clear, perspicuous, lucid are comparable, as used in reference to qualities of thought or style with the meaning, quickly and easily understood.
Clear implies freedom from obscurity, ambiguity, or the danger of being misunderstood.
Perspicuous lays more stress than clear upon the medium of expression regarded for itself; it frequently connotes a certain simplicity and elegance of style.
Lucid especially implies clearness of order or arrangement.