hedwig, do you mean you want to clip grasses and weeds? Are you wanting them for mulch, or you want them to break down in place?
One thing you can do is put a dark tarp over them for varying periods of time which blocks out the light. If you want to knock them down by half, cover them for a couple of weeks, check every few days to see how it responds. They'll get pale, but they'll come back at about half the height. If you want to break them all the way down to brown, leave the tarp on until they get that way, maybe a month? The wetter they are under that tarp, the faster they break down.
If you have small spaces you are needing to clip, you can put down soaked cardboard which also blocks the light. Keep it wet so it won't curl up and let in light.
I always let my clovers get as tall as they want, because I need them to go to seed. If I have to plant among them, I rip off a row's worth by hand, using that for mulch, and plant right in among the clovers. I make sure the clovers don't grow over the new seedlings. And I've found that now that I keep the grass away, the clovers are winning
"Life flows on within you and without you"...George Harrison
Coastal California, USA, Mediterranean climate - no summer rain, a little frost mid-winter