c. 1300, of vision and speech, "in a clear manner, without obscurity," from clear (adj.) + -ly (2). Meaning "evidently" is from 1560s; as a parenthetical expression in argument, "it is clear," recorded from 1867.
history has clearly shown the folly of that policy
they were clearly lost
I could clearly see myself in his situation
could be seen clearly under the microscope