The diaphragm or as some call it the pressure plate, is the outer part of the clutch, with the sprung fingers in it, the bit that actually clamps the friction disc between the flywheel and clutch diaphragm.
The diaphragm is inside the bell housing so will be a box off job, first though make sure it is not over adjusted by some numpty.
PULL on the clutch pedal, ie up past it's normal(rest) position, this will release any tension in the adjuster, then a few presses later will auto adjust to correct itself try that first.
tabetha