Tree in border?

Safflower

I have really neglected my border in the back garden and now it's an overgrown mess! I think this is a tree that is growing in it but what is it? Also how do I get rid of it? It looks like it would be difficult to remove given the size of the trunk. Any suggestions?

Safflower

Thanks! There is a wood behind my garden so I guess the trees are self-sowing lol. What about the roots though? You think I could dig them up?

Safflower

I'll let you know how it goes.