There can be some problems with feeding clover, whether in pasture or hay. Here is article on such. Grass For Horses|Best Grass Safe Grass Horse Pasture|Horsepasture.com Clover can be a desirable feed source for most horses whether used in pasture or in hay because it provides useful energy and acceptable protein and fiber. Clover also is a natural nitrogen builder for soil, eliminating the need for expensive and environmentally detrimental chemical nitrogen fertilizers.
Clovers may occasionally be infected with mold, causing slobbers, photosensitivity, and bleeding. Even with these potential problems, clover is still considered a useful forage for horses. Only moldy clover can cause toxicity problems with horses. In very wet years or periods of high humidity, fencing horses out of clover rich pastures is probably the best control strategy.
To decrease the chance of mold, you can increase air movement, by mowing, thinning clover stands, or improving drainage. When using clover for hay, keep in mind that clover, especially red clover, takes longer to dry than other forage species.
For more information and a pictorial view of the different species of clover see this article on feeding horses clover
and this article from Penn State on Pasture and Hay For Hor
Another article on Photosensitization related to alsike clover ingestion, which I am now dealing with in my grade Paint. Alsike Clover Toxicity in Horses - Forage ? British Columbia Ministry of Agriculture Grazing Dangers | The Chronicle of the Horse
As to hours out? Mid-morning after dew is off grass good, and a couple of hours only. Depending on number of horses it may well be grazed down soon anyway, or even bare lot if horses are left on it too long.
But if you can get the clover out of there before next year and plant it with something different, I sure would.