I have completed the grid, apart from the final letter at 15a in the grid. I have got there by solving the clues on a separate sheet then jig-sawing the answers into the grid.
However, I have the same trouble as you, Eretrep, in understanding the preamble in order to insert the final letter and compose a clue at the asterisk! I don't understand what is meant in the second line of the instructions by "where it stands", and often the definition appears to be more than one word.
Help, please, anyone who is also tackling this one!