The across clues are normal.
In each of the down clues, the definition has been moved into the subsequent clue.  I.e. The definition for 1d is in the clue for 2d, and that for 2d is in the clue for 3d etc. The definition for the final down clue (23d) is in 1d.  This may sound daunting to start with, but it should become less so as you progress.  Once you solve a down clue, it helps you with the clue either side of it.  (You will know the definition for the preceding answer, and will be able to locate the definition of your solved answer within the succeeding clue.)