eenk
4Dn is a C clue. 12 Dn is a D clue
The logical working out of how certain jumbled entries (A Clues) had to be, and then arriving at the 10 letter phrase (4 2 4) reading across from left to right, gave me my entry into how to arrive at the thematic word.
For instance, the first letter has to be from 2Dn which is an A clue, and because 25, 28, 32, 34, 36 Ac are all entered normally i.e. they are NOT A clues.
Once you have the letters of 2Dn, you should know which letter the wordplay has omitted. Logic dictates that the entry at 2 DN has certain letters already given by crossing entries e.g the first, second, and sixth. The 4th letter has to be shared with the entry at 13Ac. So, you can narrow down the choices for what goes at column 2 row 5. It has to be a letter shared with 15Ac.
Similarly, you can work away at Column 3 Row 4. It has to be a letters shared by the chariot and intend (both A clues) AND further indicated by how A clues work. Only one letter will fit there. If you continue this process across the page, you will see that the choices are very limited, ESPECIALLY if A clues are involved, because you know it has to be the letter omitted by the wordplay.
Once you have the (4 2 4) phrase. The letters which overwrite it are dictated by the fact that real words must appear. The first 2 are blindingly obvious, and then the third becomes so. In order that 4Dn makes sense, only 3 letters can go there A, D or T. Only one letter will fit in 20 Dn and 7 Down . You end up with a word which I did not recognise, but I was helped out by the first 6 D clue entries. Read them out loud quickly.