It took me forever to find it, too. But the item that looks like a broken mirror is it.
There is no cheat in any mission in any 39 clues game!
mission 7 39 clues answer for here lies b. fore e . fines
arctic
On a bookshelf at the top
yep
Boxes on Beechey Isle
456
good
Arctic is the code you have to type in.
Near the bookcase on the right.
there is no magic box
there arent any