Like jim says, it would be "wrong" to give the game away explicitly on this one - there is a fantastic feeling of elation when you manage to crack this one!
Think about grid symmetry - you know there is a 1a and a 1D, so there must be corresponding answers diametrically opposite. So even though 1A is unclued you know its length and you can work out which answer corresponds to 1D. Then look at how the last few downs might slot together in the SE corner, remembering you need a similar pattern in the NW to go with 1A and 1D. The unclued 1A should start to take shape and then you might see how the whole grid is coming together.
A very nice example of carte blanche!