what is it and how do i get rid of it?

Does your neighbour have any trees in their garden? It looks very like a tree seedling as I have this problem with trees outside my boundary fence. They seed themselves everywhere and have tough little stems.

If the leaf matches any neighbouring trees then you will have a little forest soon if you don't pull them up! I have one that I missed and it is going to be a huge nuisance now to get rid of.

Is it thorny? Looks a bit like bramble.

If they are seedlings pull them up when you see them appear first before they get tough to pull, they look like tree seedlings, or are they runners coming up off the roots ?The populus Canescens tree leaves look simular to those of yours, if so I would use a bush spray and make sure you completely wipe them out, A Neighbour of ours had a Populus "Candicans Aurora" when we lived in the town for three years, I was glad to move they were coming up through the lawn everywhere. They grow rapidly with a very long root run and they say do not plant within 130Ft of a building or drains as they will do disasterous damage. It is a tree that a lot fall for especially the "Candicans Aurora" which has coloured leaves green cream and pink and in the evening light the pink shows up like lit candles, you can see why people fall for them. You will be able to identify it from the tree next door. The Canescens is plain green and more rounded with a serated edge.

Yes...looks very much like Bramble...

i have this problem in my garden with a tree that comes from next door. i missed one a few years back that decided to grow right in the middle of another bush. the dam thing grows faster than a conifer and because my garden was neglected for a while it now has to stay as it is as tall as the house. too big for me to handle and cant afford to get someone in to do it. i now check in between all my plants and seem to be pulling them up daily.