Many thanks Ged for your explanation. I wrote my comment before I'd read pages 2 and 3 on the forum. I shall now try to work out which clues need which letters added. I gather that I add the letter before doing the anagram. I assume there is a definition for the word before you add the letter and then anagram a word that is undefined.
I think I've got it now. I just need to see if I can actually solve any clues.
P.S. Why does tag mean party in 17d?