Complex sentence: Dumbledore agreed and ensured that they were placed under the Fidelius Charm, and in return, Snape secretly allied himself with Dumbledore and the Order of the Phoenix as a re-doubled agent against Voldemort, using his powers of Occlumency to hide his betrayal from Voldemort.
Split into simpler sentences: Dumbledore agreed and ensured that they were placed under the Fidelius Charm. In return, Snape became a re-doubled agent for the Order of the Phoenix against Voldemort, using his powers of Occlumency to hide his betrayal from Voldemort.