Normally they should have indicated any damage to the car to you when you returned it - otherwise you have no way of proving that the damage was done after you returned it. I've never heard of anyone being billed afterwards for damage. You should read the small print on the rental contract but if I were you I would contest it.
Unless they have proof that you used the car outside the terms of the rental agreement (eg for a motor race, rally, hillclimb) or that you fitted a temporary hitch and used it to tow large loads, I can't see how you are liable, but you'll need to check the hire contract.
Never come across that before. Agree with above, contest it. Things like clutches would surly be damaged from ware and tear over time, how can they prove the damage was done by you or anybody else for that matter when different people are driving it every couple of days?