Is it possible to override git command by git alias?

It is NOT POSSIBLE

This is from my clone of git.git:

static int run_argv(int *argcp, const char ***argv)
{
    int done_alias = 0;

    while (1) {
        /* See if it's an internal command */
        handle_internal_command(*argcp, *argv);

        /* .. then try the external ones */
        execv_dashed_external(*argv);

        /* It could be an alias -- this works around the insanity
         * of overriding "git log" with "git show" by having
         * alias.log = show
         */
        if (done_alias || !handle_alias(argcp, argv))
            break;
        done_alias = 1;
    }

    return done_alias;
}

So its not possible. (handle_internal_command calls exit if it finds the command).

You could fix this in your sources by changing the order of the lines and making handle_alias call exit if it finds the alias.

As already mentioned, it is not possible to use a git alias to override a git command. However, it is possible to override a git command using a shell alias. For any POSIXy shell (i.e. not MS cmd), write a simple executable script that performs the desired modified behavior and set a shell alias. In my .bashrc (Linux) and .bash_profile (Mac) I have

export PATH="~/bin:$PATH"
...
alias git='my-git'

In my ~/bin folder I have an executable Perl script called my-git that checks if the first argument (i.e. the git command) is clone. It looks essentially like this:

#!/usr/bin/env perl
use strict;
use warnings;
my $path_to_git = '/usr/local/bin/git';
exit(system($path_to_git, @ARGV))
    if @ARGV < 2 or $ARGV[0] ne 'clone';
# Override git-clone here...

Mine is a little more configurable, but you get the idea.

I opted to solve this with a bash function. If I call git clone, it will redirect the call to git cl, which is my alias with some added switches.

function git {
  if [[ "$1" == "clone" && "$@" != *"--help"* ]]; then
    shift 1
    command git cl "$@"
  else
    command git "$@"
  fi
}