I purchased an injection needle that can be plugged into the end of the grease gun fitting.
You can find this accessory at most auto parts stores near the grease guns.  Careful, it is sharp. 
 
From the interior insert the needle a little bit along side of the clutch cable.  Wrap the opening with a paper towel and apply finger pressure to seal off the opening. 
Pump some grease into the clutch cable tube.   It could get a bit messy! 
 
Clean up the excess grease that squirted around the opening, done!
 
It smoothes the clutch action and helps increase the clutch cable life.