this.offset is not a function within a click function

Replace these two lines:

var topx = _this.offset().top;
var leftx = _this.offset().left;

with:

var topx = _this.offsetTop;
var leftx = _this.offsetLeft;

As .offset() is a jquery function and _this is a DOM element.

---

Also for your .css line you have to wrap _this in $(...) again because _this is a DOM element and **not a jQuery object.

this (and hence _this) inside your event handler refers to a DOMElement which doesn't have the offset() method as that's part of jQuery. To fix this you can create a jQuery object using $(this):

$('.item').click(function(e) {
    var $this = $(this);
    var topx = $this.offset().top;
    var leftx = $this.offset().left;
    var moveArea = $('#replace').offset().top;
    var moveLeft = $('#replace').offset().left;
    var moveUp = topx - moveArea - 50;

    $this.css({
        'position': 'absolute',
        'top': moveUp,
        'zIndex': 50,
        'left': leftx
    }).animate({
        top: -50,
        left: moveLeft
    }, 300)
});

Also note the use of the object provided to a single css() call over multiple calls to the same method.