I'd say it's a young elder (Sambucus nigra) aka elderberry tree. Seed probably deposited by a bird. They grow quickly and are really best suited as part of a field hedgerow rather than a garden. Once they get established, they are harder to remove.
If you crush a leaf then it will smell quite strongly and not very nice, if it is elder.
A trowel in the hand is worth a thousand lost under a bush.
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