There are two distinct themes Nikki, connected by a relationship between members within each theme which is then used to reallocate answers to wordplay to the correct place in the grid.
For example the wordplay at 7A leads to an answer to be input elsewhere. The wordplay at this other location relates to an answer from the second theme which is input at 7A. The two (one from each theme) are connected in a consistent way.
The extra letter of these 12 clues goes where the word is entered not where the wordplay appears, to spell out the description.