These would be a lot easier if the "operations" applied to all clues, not just some (of course).
I have almost finished (three or four to go), but it seems 23D needs one letter added to the clue AND the same letter added to the solution. Surely not?
The fourth letter of 21A seems a little arbitrary, but must be 'S' I thought, until I realised the answer is 2-2, not 4. So is the fourth letter O?
And nothing seems to fit 9A and 17D! I'm still working on those.