Clearly means in a way that is easy to see or understand. Speak clearly, clearly marked, clearly wrong.
As a sentence adverb it asserts obviousness — clearly he was lying — and that use is a warning sign in argument: writers reach for clearly exactly where the point is not clear, which is why editors strike it out. As a manner adverb it is entirely useful. Note clear, clarity.
1adverbclearlyIn a way that is easy to see or understand.
“Please speak clearly so that everyone at the back can hear you.”
2adverbobviouslyObviously.
“She was clearly upset, but she kept saying she was fine.”
Too few real examples of this word to estimate which sense is commonest.
clearly is a manner of plainness.
Clear plus -ly, from Old French cler, from Latin clarus, bright. Clarify and declare come from the same brightness.
Middle English
clerli
English, today
clearly
From Middle English clerli, clerely, clerelych, cleerliche, clerliche. by surface analysis, clear + -ly.
Etymology from Wiktionary, CC BY-SA