Complex sentence: At the time of her accession, the government was led by the Whig prime minister Lord Melbourne, who at once became a powerful influence on the politically inexperienced Queen, who relied on him for advice.
Split into simpler sentences: At the time of her accession, the government was led by the Whig prime minister Lord Melbourne. The prime minister at once became a powerful influence on the politically inexperienced Queen, who relied on him for advice.