If ATC has cleared you to cross MARRC intersection at FL180, do you need to adhere to this once given a descent clearance by a new controller?

The statement that the latest clearance has precedence is correct. In aviation procedures, when you receive a new clearance from air traffic control (ATC), that clearance overrides any previous instructions. This means that if a new controller gives you a descent clearance after you have been instructed to cross a specific intersection at a certain altitude, you follow the new clearance instead.

ATC often updates pilots on their clearances due to changes in traffic, weather, or routing needs, and it's vital for flight safety that pilots adhere to the most current instructions. The principle here is that safety and effective air traffic management rely on ensuring pilots act according to the most relevant information available.

Understanding that clearances build on one another helps pilots navigate safely and compliantly throughout their flight. While the initial clearance to cross MARRC at FL180 might have been issued, receiving a new directive entails that you must act according to that new information.