Help would be welcome for downs starting Wide, fast and Protozoan.
Also re final answer, not in Chambers, am I right in thinking definition is either solar OR disc as one is extra?
That had me worried for ages, since I was thinking that it must be the first two letters of a down solution and nothing would work. Serves me right for not taking note of every word in the rubric.
Lumen, the definition is solar disc, and for the other 3, delete the first letters in part of the clue, and the protozoan is 3 letters for a truncated period in side a 4 letter word for name
The wiki entry has a chart showing all three variations of the system - the relevant one has some vertical lines with letter codes on one side or the other (there are some vertical bars in the crossword grid...).