50% Golden Hammer

Yes you can clean slate it (if its the same as in kMS).

The way the 50% Golden Hammer works is that it adds a slot to the item no matter the outcome of the hammer. If the hammer passes, you have a slot ready to use, but if you fail, the new slot gets instantly used up. That's why you can clean slate the slot back. Since one of your hammer slots got taken up, I'm gonna assume it works the same way in GMS.

I can validate that this is how it works in GMS. I hammered my glove and it worked. I hammer the second time and it failed. The slot was used but not available. I used the clean slate scroll and when it did pass, I was given the hammered slot and was able to use my scroll.