Simplify the following text for a Grade 8 native English-speaking person.
Original: After six years under Sylvester , he spent a final year of apprenticeship in the New York City office of William P. Van Ness .
Simplified: After six years under Sylvester , he spent a final year of apprenticeship in the New York City office of William P. Van Ness , a political lieutenant of Aaron Burr .