After deleting the bash completion stuff and replacing the verbose description with Guix's, it cut the package from 63 lines down to 36. Deleting the blank lines cut it down to a further 27. For comparison the Guix package (which had no blank lines to begin with) is 35 lines.

Here's the trimmed Nix derivation:

  { stdenv, fetchFromGitHub, autoreconfHook, ncurses, libevent, pkgconfig, makeWrapper }:
  stdenv.mkDerivation rec {
    pname = "tmux";
    version = "2.9a";
    outputs = [ "out" "man" ];
    src = fetchFromGitHub {
      owner = pname;
      repo = pname;
      rev = version;
      sha256 = "040plbgxlz14q5p0p3wapr576jbirwripmsjyq3g1nxh76jh1ipg";
    };
    nativeBuildInputs = [ pkgconfig autoreconfHook ];
    buildInputs = [ ncurses libevent makeWrapper ];
    meta = {
      homepage = http://tmux.github.io/;
      description = "Terminal multiplexer";
      longDescription =
        '' tmux is a terminal multiplexer: it enables a number of terminals (or
  windows), each running a separate program, to be created, accessed, and
  controlled from a single screen.  tmux may be detached from a screen and
  continue running in the background, then later reattached.
        '';
      license = stdenv.lib.licenses.bsd3;
      platforms = stdenv.lib.platforms.unix;
      maintainers = with stdenv.lib.maintainers; [ thammers fpletz ];
    };
  }