$.fn.flex = function ( options ) {
   var p = this.data("flex"),
           opts = options || {};
   if (p) return p;
   this.each(function () {
      p = new Flex( this, opts );
      $(this).data("flex", p);
   });
   return opts.api ? p : this;
};

<script type="text/javascript">
    $(function() {
        $(".flex").flex();
    });
    </script>

TypeError: jQuery(...).flex is not a function

<script src="/path/to/jquery.js"></script>
<script src="/path/to/the-plugin.js"></script>

foo = function () {
  // this is a minimal function
};

$.fn.flex = function ( options ) {
    // ideally some code here
};