As far as I can tell they already have clutch slippage simulated, but it's not easy to encounter. I know AR162B's Moonhawk mod features an engine that is too powerful for any of the manual transmissions, causing the clutch to never grab, and I don't think he took the time to add that from scratch (which implies beamng already had it). This topic has been broached on this thread (and probably a few others) already http://www.beamng.com/threads/will-beamng-have-actual-engines.224/page-2. At the bottom of that page I wrote a tome of info on drive-train failures, so if you are unfamiliar with clutch or other running gear failures you could skim through it to get a feel of how beamng would likely handle them. To start though, all of the cars in Beamng are made to drive like factory new vehicles, and factory new vehicle tend to have pretty resilient clutches with plenty of grab for their power outputs.
