Usually a dry clutch release bearing, dry pedal pivot, or worn pedal bushes. None are dangerous immediately, but a noisy release bearing is an early warning of clutch wear.
Here's a quick, honest answer from a working UK mechanic — what causes it, what it costs to fix, and what to do next.
How long can I drive with it? — Months usually — but plan the repair. A failing bearing can damage the clutch fork.
Can WD-40 fix it? — Only on the pedal pivot, never inside the bellhousing — oil contamination ruins the friction plate.