I'm not clear on precisely what you are after, or just what the conditions are. Questions/'guesses', looking for clarification.

You are looking for a ground cover that will tolerate being mowed, say, late spring and early fall? Doesn't matter if it looks ugly-ish for a while, as long as it recoups and thrives through the rest of the season, and comes back healthy?

An old windbreak.... How wide is the windbreak? 10 feet? 50 ft? 100 ft? And how is the windbreak laid out? If it was simply a windbreak to block winter winds from a house, it might have been planted roughly northeast-southwest, and would have fairly even light to moderate shade under those high branches. If it was a agricultural/garden windbreak, it might have been planted pretty much north-south, and if fairly narrow, might get much heavier sun in late afternoon.... not long after the heat of the day.

"A legume" is unlikely to be possible. Were you asking about that for the nitrogen addition to the soil? Or for some other reason?

There are a number of possibilities, I think. Let us know more about the conditions. A picture of the area wouldn't hurt....

Clover?

I agree we may need more info. Also, have you checked with you local extension service?

tj

Here is a link that might be useful: Illini extension