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?