1. Stand next to the thing you want to clip into;
2. Start moving to the direction opposite of the thing that you want to clip into (so if it's on the right you start moving to the left (note: this doesn't work if the thing is above you, hence the (almost) in the title));
3. As soon as you start moving press the key opposite of where you are moving, so if you started moving to the left you press the right key;
4. Congratulations, you managed to clip into a wall/fruit/block/knocked out enemy, you can now move to any free adjacent tile!

Note 1: You can't clip upwards into anything for some reason. Only left, right, and down.

Note 2: If you eat a block before doing this and then spit it out while you are clipped you will effectively remove the tile you were on from the game, only its sprite will remain and you wont be able to interact with it in any way.

Note 3: This should be obvious, but in case it isn't: if there's a solid block in the opposite direction of where you want to move, you can't clip. You actually need to start the movement and then cancel it for a successful clip.

Credit goes to SlushPuppy for finding this awesome glitch.