An alternative sentence: Cabin crew - the people who serve travellers on board aircraft, trains and ships (Sorry, I'm not sure what you mean by 'in the way')

Since its crew, it should be the "people that serve". "In the way" sounds pretty strange. I'll assume you're trying to say "down the aisle". So... the definition would be something like "Cabin crew: the people who serve customers on airplanes, trains and ships down the aisle". I've changed "of" to "on" because it just sounds slightly weird, I don't think it's grammatically incorrect, but it definitely does sounds strange, apart from the fact that saying "customers of airplanes, trains, and ships" could mean people who buy airplanes, trains, and ships.