What plant ?

Peter Livesey

This has recently appeared in the corner of a bed. Please could someone let me know if it is a weed

Hostafan1

looks like a dahlia to me.

fidgetbones

One of the bishop series of dahlias.  Unlikely to flower this year but if you let the frost take the top off, dig it up and keep over winter frost free and have an early start next year.