Driving along nice and easy into third back off the clutch then clunk and two clicks of the ratchet, repeat the latar for each time clutch pedel is depressed!

I've had a look under there and nothing is obviously fooked so what is the usual bit thats fooked? Whats needs to be done, what needs to be changed?

Car is a mk1 fiesta but i'm 9o% sure it's the same accross the range.