So by pressing Ctrl+K you can clear the input area but it really annoys. The hotkey isn't listed in miranda Customize->Hotkeys configuration page and you cannot disable it. Using this hotkey doesn't allow you to restore written text - you can't undo (Ctrl+Z) it. Also the requirement of this hotkey is under question - you can always press Ctrl+A + Delete and this is a default behaviour for most OSes and you can also restore the text by pressing Ctrl+Z. It's easy to hit Ctrl+K if you type fast while using Ctrl+Left/Right arrow-keys to step between words and have your message lost and unrecoverable.

this undocumented hotkey can also have conflict with other bindings.