The answer to this riddle has a hole in the middle, And some...

The answer to this riddle has a hole in the middle, And some have been known to fall in it. In tennis it's nothing, but it can be received, And sometimes a person may win it. Though not seen or heard it may be perceived, Like princes or bees it's in clover. The answer to this riddle has a hole in the middle, And without it one cannot start over.