If the dealer has said your previous vehicle has been clocked, we would ask them to provide the evidence to show this. A look at the MoT history of the vehicle or paying for a vehicle history check should provide the mileage of the vehicle over its lifetime, and if a mileage change is shown then this would suggest it has been changed in the past. If this is the case then it depends on whether the dealer will still accept your vehicle as part exchange - if they will not, then unfortunately you will have to return the vehicle and sell your previous vehicle privately, although you will need to declare the mileage change when selling.