If it is a hydraulic clutch release mechanism, you may have to rebuild the master cylinder. The brake fluid may be leaking past the seals in the master cylinder. It doesn't leak out but is returned to the pressure side of the master cylinder on the return stroke. This is the clutch master cylinder I am talking about not the brake master cylinder.
If the the pedal is held down for a while and the clutch starts to engage (and you are not losing brake fluid), this is a sure sign of leakage in master cylinder.
A repair kit is available and the only special tools you need is a cylinder hone. Its not a difficult job.
If you have to add fluid often to the reservoir then the slave cylinder must be rebuilt also.
P E H