late 13c., "bright," from Old French cler "clear" (of sight and hearing), "light, bright, shining; sparse" (12c., Modern French clair), from Latin clarus "clear, loud," of sounds; figuratively "manifest, plain, evident," in transferred use, of sights, "bright, distinct;" also "illustrious, famous, glorious" (source of Italian chiaro, Spanish claro), from PIE *kle-ro-, from root *kele- (2) "to shout" (see claim (v.)).
The sense evolution involves an identification of the spreading of sound and the spreading of light (cf. English loud, used of colors; German hell "clear, bright, shining," of pitch, "distinct, ringing, high"). Of complexion, from c.1300; of the weather, from late 14c.; of meanings or explanations, "manifest to the mind, comprehensible," c.1300. (An Old English word for this was sweotol "distinct, clear, evident.") Sense of "free from encumbrance," apparently nautical, developed c.1500. Phrase in the clear attested from 1715. Clear-sighted is from 1580s (clear-eyed is from 1529s); clear-headed is from 1709.