By 'last level' do you mean when you are in your real house and you're winding the last clock in the last room to bring about the sunrise? And by 'full' do you mean that the face of the clock no longer has any grey segments on it? Because once you wind the clock for the last time in your real house, the game ends, and depending on whether the girl or the monster is in the background, you either get the Pagurian ending or the Breaking the circle ending.
In the last level the clock isn't "full". Sometimes when you spin it, it still isn't full. Doing all these things fills the clock at the last level ensuring that when you activate it that it will result in a full circle. I've heard that doing so results in getting the third ending.