Redefining 'def' is a trick used in Tcl to make "safe" configuration languages exposed to the end user. You define all of your configuration options as functions and then undefine everything else (including "proc" which corresponds to defun) allowing you to evaluate users' config files with this safe interpreter as code.

I can't find a good source for this practice of manually undefining functions, but here is a link to relevant built-in functionality: https://www.tcl-lang.org/man/tcl8.6/TclCmd/safe.htm