connecting clutch line too. Most likely your problem is in the clutch line and the line is leaking fluid when you depress the clutch in to change gears. Get a new Nissan slave cylinder and a aftermarket SS braided clutch line along with some new DOT 4 brake fluid.
Also when you change the slave cylinder you will end up draining the trans manual gear fluid as the slave cylinder bolts directly to the transmission. Good time to replace that fluid too.