What is the answer to this riddle?

I just noticed several riddles, so I decided to post this one. ANSWER IT!!!

"The answer to this riddle has a hole in the middle,
And some have been known to fall in it.
In tênis it's nothing, but it can be received,
And sometimes a person may win it.
Though not seen or heard it may be percieved,
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."