https://www.mobileread.com/forums/sh...d.php?t=358795
There's a post there with Oxford Dictionary. Shorter one is as detailed as default Kindle Oxford Dictionary. Longer one is overkill.
When you have at least one dictionary installed, long press on a word will start it up with a popup definition of the word that was pressed. Along it, you'll also have options to move from one dictionary to the other.