citreon xsara picasso 2.0 hdi sko2 ccx.stop lamp comes on when clutch pedal is depressed

We have no references to this vehicle. The brake light circuit is shorted to the clutch switch output circuit-

Probably as the answer above if you mean the brake lights come on. If you mean the STOP light on the dash that is a different thing all together. Phase 2 Xsaras have multiplexed wiring which is prone to French gremlins. If it is the STOP dash light and only on when pressing the clutch then sounds like a short somewhere. Make sure you check coolant levels as the STOP light is usually an indication of overheating.