You may find an easy way to do this is with a conditional torque that would act on the gear only if the rotational velocity is either negative or positive, depending on which way you wish your gear to freely rotate. This way, you can set a limit to the amount of torque you wish the clutch to handle. Do this by first creating a measure of the joint's rotational velocity. Then, create a torque on this joint which will act only in the specified "domain" related to the rotational velocity. This will allow your gear to rotate freely in one direction and "lock-up" in the other (limited by the torque you choose to apply). I haven't yet tried something like this, so I'm tossing this out off the top of my head, but I'm sure it will work based on other similar models. I hope this helps ...
Cheers,
Kaz Z06