Not quite a function but a small program: xdotool, that simulates keyboard and mouse activity. For plenty games xdotool and bash are all you need for an amazing (albeit simple) autoclicker. For example:

# Clicks the screen every 40ms. Great for Cookie Clicker and similar games.
if [[ $(xset -q | grep -o "Scroll Lock: off") == "Scroll Lock: off" ]]; then xdotool key Scroll_Lock; fi
  ScrollStatus=$"Scroll Lock: on"
  while [[ $(xset -q | grep -o "Scroll Lock: on") == "Scroll Lock: on" ]]; do
    xdotool click --repeat 4 --delay 40 1
    done

# Goes up, then down, then presses C. I use it to farm BP in Final Fantasy V, in an emulator.
if [[ $(xset -q | grep -o "Scroll Lock: off") == "Scroll Lock: off" ]]; then xdotool key Scroll_Lock; fi
  ScrollStatus=$"Scroll Lock: on"
  while [[ $(xset -q | grep -o "Scroll Lock: on") == "Scroll Lock: on" ]]; do
    xdotool key Up; xdotool key Down; xdotool key c
    done

This you assign the script to a shortcut, press it to turn it on, and Scroll Lock to turn it off.

It gets even better - with grabc (another small program, that probes the colour of a pixel), you can even make autoclicking scripts that react to changes in the game screen.

EDIT: as Tal highlighted, xdotool is for X11. It works with Wayland… a bit, most functions are broken. Wayland users are better off using ydotool. Same basic idea, though - you don’t need to click it, you can tell your computer to do it for you!