Clutch fluid? - hahahahaha
Likely the clutch is worn. Also, it's possible that the cable is stretched. (For the record, the Mustang's clutch is not hydraulic, but cable operated.)
I'd start by replacing the cable and add a firewall adjuster and aftermarket quad. Steeda makes an excellent kit. Even if it turns out to be a worn clutch, you'll be glad you have these new pieces already installed with a new clutch.