What is a clean catch of urine and how important is it in a 24 hour urine exam for protein?

Two different things. Clean midstream catch is a term used for obtaining urine that is after the stream has started. It's usually a one time catch, or serial one time catches. The 24 hour urine sample requires you to collect all urine and 24 hours. That's used to determine how much of a substance has been peed out over 24 hours, and dividing it by the milligrams that was collected from what was ingested.