I don't think the cable is stretching.
Most likely, the big adjusting "wing nut" is backing off. It happens more than people expect.
The easiest cure is to remove the wing nut and take it to an ACE Hardware store to match the threads to a regular (non-wing) nut - It's metric, but I can't remember the size or pitch off the top of my head so just match it.
Go home and put the wing nut back, adjust your clutch for 3/4" - 1" play at the top of the pedal and thread the regular nut on after the wing nut and use it as a locknut - Just crank them together nice and tight. Bonus points if you use a Nylok insert nut as the locknut, but that's not absolutely necessary.
That should cure it.