Trying to create an overlay for plasma-framework

Hi all, I’m trying to use a custom fork of plasma-framework (I have an issue with an xmonad / kde plasma-5 desktop configuration that requires a manual patch).

From what I can tell, the source of plasma-framework in nixpkgs is in libsForQt5.kdeFrameworks.plasma-framework which is configured in nixpkgs/pkgs/development/libraries/kde-frameworks/default.nix. The code in there is a bit beyond me, but from what I gathered I needed to set the attribute src.plasma-framework. So I wrote up the following little overlay

    plasma-framework-overly = (final: prev: {
      libsForQt5.kdeFrameworks = prev.libsForQt5.kdeFrameworks.overrideAttrs (oldAttrs:
        rec {
          plasma-framework.src = pkgs.fetchurl {
            url = "";
            sha256 = "05q0hw64snmkqs2sl81g2l9ak4sjriwgsd2b7xkp9bxlr3n4wm00";
            name = "plasma-framework-5.87.0.tar.xz";

However when I go to compile I get a error: attribute 'callPackage' missing. What am I doing wrong here?