That would translate into "That is a professional work", which sounds pretty odd to me. With det, you already specified which work you meant, so there is no need for the et, at least I see it like this
Policemen say that all the time while they stare at a corpse in black movies...
Does it mean that this work has been done perfect or that this work's been done by professional (which is pretty the same but i would say there's the difference) or that this work can be done only by professional?
And does "arbejde" is being used in the meaning of "piece of work"?
It could mean that the work is well done, that the work was done by professionals or that you paid a lot of money for a ❤❤❤❤❤❤ piece of work (by adding sarcasm). It could also be used to describe future work that you expect to require a certain level of skills -- on the level of a good professional. Usually it just means that the work was done well.