The term 'smoking gun, so to speak' is a metaphor used to describe something that serves as undeniable PROOF of guilt or wrongdoing. The crossword solution for this term is PROOF, which is a five-letter word. This phrase is often used in legal contexts to refer to evidence that is so strong that it cannot be denied or argued against. It is also used more generally to refer to any kind of evidence that is so convincing that it cannot be disputed.