What is a clean offer, otherwise known as a firm offer in a seller's market? You might hear this term thrown around by either sellers or buyer's agents, and it's a term that's used when an offer is submitted without any conditions. It's also considered a contingency offer. Which could be the reason as to why another offer could fall through.
Now, this isn't to say that that clean offer is a firm guarantee, but the likelihood of any issues happening on close are less likely to happen.