Complex sentence: The advance of the Anzac Mounted Division was held up at the Tel el Saba redoubt--by the time it was captured the attack was running many hours behind schedule and the possibility of launching the combined infantry and mounted assault on the town before nightfall looked slim.
Split into simpler sentences: The advance of the Anzac Mounted Division was held up at the Tel el Saba redoubt. By the time the redoubt was captured, the attack was running many hours behind schedule and the possibility of launching the combined infantry and mounted assault on the town before nightfall looked slim.