Bring to naught is an idiom which means to reduce something to nothing or to make something completely ineffective. The crossword solutions for this term are ANNUL (5 letters), UNDO (4 letters) and RUIN (4 letters). ANNUL means to declare something legally invalid, UNDO means to reverse the effects of something and RUIN means to destroy something. Therefore, the three solutions all refer to the same concept of reducing something to nothing or making it completely ineffective.