What term you use depends on what you are describing.
Clear = when you have a field or text area in which you want to clear all the content.
Reset = when you have a default setting or settings and text and would like to return it to its default state.
Cancel = when you have made changes to settings or data and you want to cancel or stop those changes without saving them.
I suspect from your question that you are talking about single characters. If that is the case, you should use "delete" or "backspace" to describe them. However if the people that you're dealing with have very little computer experience, it may be more useful to use a symbol for this. Something like the icon below.
Whatever term you use, don't use "correctness" as language wise it makes very little sense.