How can I select item with class within a DIV?

Try:

$('#mydiv').find('.myclass');

JS Fiddle demo.

Or:

$('.myclass','#mydiv');

JS Fiddle demo.

Or:

$('#mydiv .myclass');

JS Fiddle demo.

References:

• find().
• Selector context.

Good to learn from the find() documentation:

The .find() and .children() methods are similar, except that the
latter only travels a single level down the DOM tree.