Where is CLKPR located on Ubuntu 16.04 with Arduino 1.8.3
grep is the command you want:

foo@bar:~/arduino-1.8.2$ grep -r CLKPR .
Binary file ./hardware/tools/avr/avr/lib/avr35/crtattiny167.o matches
Binary file ./hardware/tools/avr/avr/lib/avr35/crtat90usb82.o matches
Binary file ./hardware/tools/avr/avr/lib/avr35/crtatmega8u2.o matches
...
Binary file ./hardware/tools/avr/avr/lib/avr5/crtat90pwm161.o matches
Binary file ./hardware/tools/avr/avr/lib/avr5/crtatmega649p.o matches
Binary file ./hardware/tools/avr/avr/lib/avr5/crtatmega168pa.o matches
./hardware/tools/avr/avr/include/avr/io90pwm216.h:#define CLKPR   _SFR_MEM8(0x61)
./hardware/tools/avr/avr/include/avr/iom329.h:#define CLKPR   _SFR_MEM8(0x61)
./hardware/tools/avr/avr/include/avr/iom165p.h:#define CLKPR   _SFR_MEM8(0x61)
...
./hardware/tools/avr/avr/include/avr/iom48p.h:#define CLKPR _SFR_MEM8(0x61)
./hardware/tools/avr/avr/include/avr/iom644pa.h:#define CLKPR _SFR_MEM8(0x61)
./hardware/tools/avr/avr/include/avr/ioa6289.h:#define CLKPR _SFR_IO8(0x3C)
./hardware/arduino/avr/bootloaders/caterina-Arduino_Robot/Caterina.h:       #define CPU_PRESCALE(n) (CLKPR = 0x80, CLKPR = (n))
./hardware/arduino/avr/bootloaders/caterina-LilyPadUSB/Caterina.h:      #define CPU_PRESCALE(n) (CLKPR = 0x80, CLKPR = (n))
./hardware/arduino/avr/bootloaders/caterina/Caterina.h:     #define CPU_PRESCALE(n) (CLKPR = 0x80, CLKPR = (n))


So it's defined in the IO header for the chip you are using (hardware/tools/avr/avr/include/avr/io[chip].h). So for the '328p that's hardware/tools/avr/avr/include/avr/iom328p.h.