Complex sentence: Lenin's answer was the creation of a party of full-time professional revolutionaries who would constitute a kind of political elite, which would possess a "correct" knowledge of revolutionary theory and act as a revolutionary vanguard.
Split into simpler sentences: Lenin's answer was the creation of a party of full-time professional revolutionaries who would constitute a kind of political elite. This elite would then possess a "correct" knowledge of revolutionary theory and act as a revolutionary vanguard.