Complex sentence: William Lithgow started his working life as an apprentice ship's draughtsman in Port Glasgow, on the River Clyde, when the Clyde was the biggest shipbuilding area in Britain.
Split into simpler sentences: William Lithgow started his working life as an apprentice ship's draughtsman in Port Glasgow, on the River Clyde. At this time the Clyde was the biggest shipbuilding area in Britain.