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--some even referred to Victoria as "Mrs. Melbourne".
Split into simpler sentences: At the time of her accession, the government was controlled by the Whig Party. The Whig Prime Minister, Lord Melbourne, at once became a powerful influence in the life of the politically inexperienced Queen, who relied on him for advice--some even referred to Victoria as "Mrs. Melbourne".