clumpy almond powder

You have to start over. No saving the clumpy batch, just keep blending that into nut butter.
To get a powder use only the pulse function and go very slowly. Might be a good idea to even fridgerate the half made batch as you go- if the almonds get too warm from the blending they release the oils to create the clumpy bits before turning into nut butter.
Your end result will be an almond meal, not as fine and fluffy as almond powder but perfectly fine to use in a lot of recipes (not so much baking cakes or macarons)

Amanda and Merrill did a tutorial on almond flour and butter a number of years ago which may be of some help:
https://food52.com/blog/2228-how-to-make-almond-meal-and-butter

It sounds like you have over-processed the almonds. I think your best bet to fix this is to call it almond paste and enjoy it on toast!

This will happen with any sort of nuts because of their high oil content. The main thing is just to take it slow and don't overblend. However, if you plan to use them with flour, mixing a bit of the flour in with the nuts before blending will help prevent clumping- you will need to account for that flour when finishing the recipe.