You are looking for wrap
method.
$('.some-class').wrap('<div id="inserted-div"></div')
$('body').prepend
-- will add a div as the first child of the body.
$('body').append
-- will add a div as the last child of the body.
But in your case you want the inserted-div
as a wrapper for an already existing div.
To target just the first instance you can either use
$('.some-class').first()
or $('.some-class:eq(0)')