I had some sort of weed/grass "fighting" me too in one of my clover plots so I hit it with Roundup last fall, killed it deader than snot. Since it was spotty and not widespread, I used my hand sprayer and selectively hit the spots I wanted to kill, getting only a little bit of the surrounding good clover.
I then frost seeded in early March over these spots and my new clover is coming on nicely, albeit quite small at this point. We'll just have to see if the weeds/grass refire or not, but judging on the brownness, if that is a word /forum/images/%%GRAEMLIN_URL%%/crazy.gif; of the area I sprayed I am going to say that I killed the bad ones and my "new" clover should be dominant again.