Planks, in each pair of down clues the removed word from one needs to be compared to the added word in the other. In each case the differences will be of these two types: (i) two single letters; (ii) a single letter and a word denoting a number. The single letters will be either one of those needed to make TENETS SURELY or A, J, Q, K denoting Ace and court cards; the numbers are the other cards below Jack. The cards are part of the thematic contest and are added to the letters in the top line to make the title.
Knowing what's needed to convert each cell in the top row helps to sort out the wordplay. For instance, I was completely baffled by 7/23, but knowing NINE was needed to turn E into N I was able to work out what had to be added to 7d and where.