If the existing grass is quite thick, the clovers will have a battle trying to grow thru all of it. If you do, you may have to mow the grass low and spray with a grass specific herbicide to get rid of the competition. I would till, hit it with roundup and then seed. You'll get a better plot, and you still have plenty of time for the clover to grow before fall. That is if the deer don't keep it clipped.

Russ