NB the wordplay comes from 10a
@Kettledrum
Twelve thematic solutions are to be entered in the grid incompletely. Their clues contain a definition of the full solution and wordplay for a different thematic entry in the grid. Wordplay for each of the remaining 23 clues contains a superfluous letter. These letters in clue order give a four-word indication of what the thematic entries are lacking. By a process of elimination, you must deduce the final thematic entry at 29 across, which (similarly completed) you might say is surprising.