There have been several discussions lately regarding the exact boundaries of Clinton Hill. To clear things up, I consulted the Best Book Ever, The Encyclopedia of NYC. Every NYer should own this enormous book. You can find answers and information on virtually everything in/about NYC. Here's the entry:

Clinton Hill. Neighborhood in northwestern Brooklyn. Overlooking Wallabout Bay and centered at Pratt Institute, it is bounded to the north by the Brooklyn-Queens Expressway, to the east by Classon Avenue, to the south by Atlantic Avenue, and to the west by Vanderbilt Avenue. The neighborhood is named for DeWitt Clinton and occupies highest ground in the area. In 1832, Clinton Avenue was laid out as a tree-lined boulevard along the crest of the hill, and some grand villas had been built by the 1840s. The area was considered a rural retreat until the speculative development in the 1860s of row houses, which by 1880 lined most of the streets and attracted affluent professionals...

The entry goes on detailing more history, but it sounds pretty defined to me: clinton-hill-map.jpg I've heard talk of the neighborhood extending one more block east to Franklin. That wouldn't be a huge stretch, but I'm sticking with the Encyclopedia.