I also have not used tcl for 400 years, but this is what I was able to put together from what I remember:
~> nix-shell -p tcl tclx
these paths will be fetched (0.11 MiB download, 0.28 MiB unpacked):
/nix/store/4lpz2cmk3lznb40vld7g9ns86zjiw1vx-tclx-8.4.1
copying path '/nix/store/4lpz2cmk3lznb40vld7g9ns86zjiw1vx-tclx-8.4.1' from 'https://cache.nixos.org'...
~> tclsh
% echo $auto_path
/nix/store/1m7mq4llm0af70w041645yx06839cwz5-tcl-8.6.9/lib/tcl8.6 /nix/store/1m7mq4llm0af70w041645yx06839cwz5-tcl-8.6.9/lib
% lappend auto_path /nix/store/4lpz2cmk3lznb40vld7g9ns86zjiw1vx-tclx-8.4.1/lib/
/nix/store/1m7mq4llm0af70w041645yx06839cwz5-tcl-8.6.9/lib/tcl8.6 /nix/store/1m7mq4llm0af70w041645yx06839cwz5-tcl-8.6.9/lib /nix/store/4lpz2cmk3lznb40vld7g9ns86zjiw1vx-tclx-8.4.1/lib/
% package require Tclx
8.4
Obviously nixpkgs should be doing this for you, so I think tcl in nixpkgs needs some work… or we’re missing something 
Maybe a tcl.withPackages 