"Bound for X" means "headed for X," "destined for X," "on the way to X." In this case, I know from cultural context that Euston is a train station. I might guess that anyway from the phrase "the 8:40 am from Euston." He is getting on a train at Euston and he plans to get off a station named Crewe. "Clearing his desk" means finishing all of the accumulated work, catching up with all of the work, completing all the work because he's going to be out of the office all next day, campaigning. "a 12-hour, five-constituency, five-speech day" means the prime minister is going to be campaigning for 12 hours, and he is going to be giving five speeches to five different "constituencies"--groups of voters. The pattern of phrase, "a 12-hour, five-constituency, five-speech day" makes the phrase "12-hour, five-contituency, five-speech" act like an adjective. Instead of "A busy day," or "A busy campaign day," or "A typical long, busy, campaign day," the writer says "a 12-hour, five-constituency, five-speech day."