Getting a window's PID by clicking on it

Yes. Try xprop and you are looking for the value of _NET_WM_PID:

xprop _NET_WM_PID | cut -d' ' -f3
{click on window}

xprop will return a window's PID.

You can filter the verbose output using awk:

xprop | awk '/PID/ {print $3}'