"They arrived at ten in the morning."
Translation:Ei au sosit la ora zece dimineața.
I wouldn't say that would be the correct way of saying it, implying that it is somehow the single or the best way for it.
It is true that given enough context we'd simplify it, but so would we in English. That's not really the point though.
Also I'm pretty sure that both la zece and la ora zece are accepted, neither being more correct than the other. However the presence of dimineață, should you choose to fully preserve the meaning of the English sentence given the lack of context, is mandatory.