Notrab, in theory there are 7 new possibilities for 1 across, but three can be discounted because they have a letter that doesn't begin any of the original clued entries. The first of the "original paired clues" gives you the first letter of the new 1 across (looking at the most likely place in the clue) and this will give you the whole word since only one of your 7 options begins with it. You then have to look closely at the following clues to see how you can get the remaining letters of the word following a systematic sequence.
Take the clues in clue order ignoring the 6 special ones. My mistake was to assume that I had to follow 10ac with its partnered clue, which is why I saw nothing for ages.