Spray it a month or so before you’re going to plant and then spray again as close as possible to planting and then drill it, you will have a fine plot.
Just my opinion but I wouldn't disc, you will just be bringing tons of buried weed seed to the surface to germinate and grow. I have planted several plots into old pastures and hay fields in the last couple years with good luck, if it's not extremely tall I would spray it a couple times with...
If it were mine I wouldn’t touch it, just let the cereals die on their own and help feed and protect your clover from the summer heat and sun that is coming.