No pressure clutch

That is the sign of a failed clutch slave cylinder or damper. The image shows the various parts. The clutch slave is on the outside of the bell housing on rear of the transmission on the drivers side. Replacement is not that difficult. You would jack up and secure the vehicle or use ramps. Set the brake and go to work.
The line gets removed, you unbolt the cylinder and bolt the new one in place. Re-connect the line and bleed the system. It could also be the damper or even a rusted line, either way the replacement is not that difficult. Easy way to discover the leak is to just add fluid and have someone slowly push the clutch down and look at the lines and parts. The leak should be readily apparent.

I read that if the clutch moves when the car is off but not when the car is on it may be the clutch?

In your case it isn't the clutch itself it is a leak in the hydraulic system that disengages the clutch to allow shifting/starting. The clutch disk and pressure plate may be worn but those are different parts than what is leaking.