IN my 88 vert with 2.3 the I was driving and I went to shift to 2nd and Instead of disengaging the pedal just went clunk!! and I took it home and the cable is still all hooked up and the adjuster I could see wasn't pulling the cable it just slipped or something.[>:] the lever underneath only pulled as much as it did before it clicked

 

Sounds like your clutch quadrant stripped. It's a freeplay adjusting item for the clutch cable. It's at the top of you clutch and brake pedals. Not the stongest part of the mustang sorry to say. The after market is full fo choices which are far stonger. It's fairly annoying to change it out too.