The Clue ___ reaction: instinct
The Answer GUT
The phrase "gut reaction" refers to an immediate response that comes from one's instincts or feelings, rather than from careful thought. This term highlights how people often trust their feelings when making quick decisions. The word "gut" connects directly to this idea, as it signifies a deep, instinctive feeling. Therefore, "GUT" is the perfect answer for the crossword clue.