I clicked on the damn thing with all the words I could think of like clue, clues, help, door, doors, room, rooms, hidden, etc. Then I gave up to look at the walkthrough and goddammit, it was... frigging retarded!
Okay, next to the door where the hanging coat is there is a lever (square empty thing in the middle), drag your box over there and leave it there. It will open the locked door. When you get in there, you will find more things. Read your items carefully and set something to the right time as like your item said. You will get the clue to another room. In this room, you will get the clue to open the safe.