How to slow ADC clock speed to 1MHz on Arduino Due?
You can change the prescaler (ADCSR register). Lowest scaler is 2, so it can never run at 20mHz. Also Atmel recommends a max ADC clock of 1MHz.

Arduino sets the ADC clock to 125 KHz (prescaler of 128)

So to match Arduino, you should select a prescaler of 8 ( set bits ADPS1 and ADPS0)