Thanks roslyn.
It was that or liquid snow.
But don't know why.
The word formed by the extra letters was important to get the bottom half.
I am assuming that the top half has something to do with the title.
S=4D?
If you've got the word formed by the extra letters, just break it up into three pieces - the first relates to the lower unclued entries, the second indicates something significant and the third (very small!) one leads you to the top unclued entries. Hope this makes sense!