**'CLKPR' was not declared in this scope**

For the life of me I can not figure out why what everybody suggests to do for slowing down the clock does not work:
I am using the Arduino IDE
Seeeduino XIAO


void setup() {
CLKPR = 0x80; // (1000 0000) enable change in clock frequency
CLKPR = 0x01; // (0000 0001) use clock division factor 2 to reduce the frequency from 16 MHz to 8 MHz

void loop() {

Error Message:
’CLKPR’ was not declared in this scope

yes - am such a beginner that green is black next to me - but I have already a few simple projects under my belt and now I want to figure out how to save energy so as I can run the XIAO from a solar pannel: first slow the clock then learn to use timed interrupts.