1
general::
Pure as the sound of a bell; also, readily understood. For example, Did you understand the message I left you?? Yes, clear as a bell. This simile, which alludes to the bell's clarity owing to lack of overtones, was already a proverb in John Ray's English Proverbs (1670).
American Heritage Idioms
2
general::
Phrase(s): *clear as a bell
very clear, as with the sound of a bell. (*Also: as ~.) • I fixed the radio, so now all the stations come in clear as a bell. • Through the wall, I could hear the neighbors talking, just as clear as a bell.
McGrawhill's American Idioms And Phrasal Verbs