Place a Clockwork Bomb on it, and a platform will appear. Destroy it with your Hobby Horse, and go inside. There's a door on the side of the wall, blocked by honeycomb. Then use the mushroom next to the keyhole to get up to the next level. Then use Shrink Sense to find a keyhole - it's above the first puzzle piece, up to a ramp - with puzzle piece number two inside. It will lead you to a puzzle, and you'll have to collect the pieces (just like you did in Chapter 2).
Then head back to the main path and continue down it.
