[nkaminar]

The grease fitting is for the housing that moves when you push in the clutch pedal and that the throw out bearing is mounted on. There is no way to grease the bearing itself. You may try to add a little heavy oil, like STP to the bearing but if it is sealed no oil will go in.

Clutch chattering is usually caused by other things such as the fingers that the throw out bearing pushes against not be adjusted correctly or grease on the clutch face or other reasons associated with the clutch itself. The added grease on the housing probably dampened the chattering but will not last, as you found out. Your best bet is to take the clutch apart and renew it, including the pilot bearing, clutch plate, and throw out bearing, and renewing the pressure plate and flywheel if they are bad. But before you do that, try adjusting the fingers that do the work. They have to be set equal within 0.002 inch, or essentially equal.