Is clover okay for my horse to eat?

In moderation. Grazing clover in hand will be fine, but leaving them to graze upon it themselves may cause laminitis as it is quite rich and it may also give them the runs as it may be a little too rich for them to graze on it all the time - a few handfuls or 5 minutes of in hand grazing wont hurt though (same principle for lush spring grass) :)