This is a dust cap that fits on the push rod bar at the bridge between the clutch hosing and primary drive. Its purpose is to stop grit and dirt entering the housing and allows the rod to keep lubricated and moving freely. Its replacement will prolong the life of the rod. See Pictures to see fitting, and dustcap in place.
 
Size:
Length:                      17mm
Outer Diameter:         18mm
Inner Diameter:          13mm
Small Hole Diameter: 4mm

Tried and tested on D7 may also fit other models, but not yet tested. 

Please note this is NOT new old stock or even a replica part. It is a part we could not originally get but it works for us so we thought we would share it.