The crossword compiler wasn't sure how to clue the word "should". Proof A trial sheet of printed material that is made to be checked and corrected. Also called proof sheet. Clue To provide someone with information which he or she lacks (often used with "in" or "up"). Smith, clue ...
In this respect it is similar to solving a crossword puzzle. One knows the length of the words and often some of the letters and usually the part of speech (from the word clue, however ambiguous). Often that alone is sufficient context to guess the word. (Interested readers who like to...