Possibly the clutch is dragging slightly. If you turn off and wait a few seconds, reverse should not clunk; if it does, it's the gear selection which causes the clunk, not the clutch.

A simple but effective trick. Since your car has no synchromesh on reverse, depress the clutch pedal and engage first gear. Then without releasing the clutch disengage first gear and select reverse. This should prevent it from crunching even if the clutch is dragging slightly.