Yes difficult my first thought plum stone had been planted or dropped and then thought apple pip dropped or planted. It seems to be large and no mention of blossom which would tie in with a stone or pip but they can take years to fruit, Or do you prune it back so no blossom can develope.
I thought it was going to be a fruiting shrub, like the adjacent Blueberry, but it’s growing very fast, like a tree. I then thought a Salix , as it’s almost under the Wiggly Willow, and has no blossoms to fruit. I now think that whatever it is, it will have to go I like the smooth, grey-green bark