The answer to this crossword question is CLAP. The clue “Respond quickly, as to criticism, with ‘back’” is referring to the act of clapping, which is a way to respond quickly and loudly to something. When someone is being criticized, they might clap back to defend themselves or show their disapproval. Therefore, the word “back” in the clue is indicating that the answer is related to a response or reaction. By combining the clues and using wordplay, we get the answer “CLAP“.