Almost always a worn outer CV (constant velocity) joint on the front driveshaft. Get it changed before the joint fails completely.
Here's a quick, honest answer from a working UK mechanic — what causes it, what it costs to fix, and what to do next.
Will it fail an MOT? — A torn CV boot is an automatic MOT fail. A clicking joint is an advisory until the boot tears.
Is it expensive? — No — caught early it is one of the cheapest driveline repairs.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.