A little water clears us of this deed meaning.

As soon as we wash the blood off of our hands, everything will be fine. No one will be able to accuse us of anything because there is no evidence.

Lady Macbeth says this in Act 2 sc 2. Macbeth is still in shock from the murder of Duncan: he gazes at his bloody hands. Lady Macbeth returns with her own bloody hands. She says that a little water will clear us of this deed: if they wash their hands, they will no longer be guilty of the crime. He statement, however, sounds desperate and rings hollow.