Don't know if I'm right aristo, but I took this as meaning an area of no development as here in UK we have Green belt areas around cities that are meant to have no buildings or industry resulting in an area of zero emissions, so it is a green zone and Zone can, I suppose mean land, so the area around the city is the green zone or maybe the Green land (two words) and it does have a question mark beside it in the clue, this I took as a way of looking at it in not the usual fashion. Now I've wrote all that, maybe it's a bit tenuous, wondering what the other guys have to say about it.