Go around the cellar in an anti-clockwise direction as far as you can, until you're facing the false wall behind the inaccessible area (you'll know you're facing it by checking the map). The switch is at the bottom of the wall about halfway across. It's almost invisible so you'll only know you've targeted it when the icon changes.
The map on the
Gamepressure guide may also help.
Edit: I'm not sure the comments about some plate on the floor are accurate. Can't say I noticed one, anyway, although I don't notice a lot of things.
